I prefer an Osprey medium sized messenger bag worn low so the stabilizer strap fits around hips like a belt, and the bag sits at the back of the hips. For commuting, it carries a roll of clothes, food, and basic tools (approx 10 lbs total).
It's low enough to not affect the center of gravity much, allows more air to the back, and it fairly aero in the drops.

I used the same bag on solo century rides last week -- one in the mountains. No problem.

Like gregf83, the weekend rides (40 - 70 mi) feel fabulous when I leave it home.

Backpack 95% of the time. The convenience of them outweighs any of the other options for me. We aren't allowed to take bags into work, so, a backpack stows easily into the work supplied lockers. Backpacks, depending on design (I use an Osprey Momentum 30), allow for much better organization of my stuff than any pannier or trunk bag. Zero theft worries because when I get off the bike, my backpack goes with me. Also, panniers add more wind resistance as my backpack is normally in my body's wind shadow. The other 5% of the time, I use my backpack and a milk crate. If someone steals the milk crate, I just 'borrow' another from behind the grocery store.

Panniers + trailer when needed. One of the bikes has no rear rack, so I use a large Carradice saddlebag with QR there.

--J

Kinda playing around with this one myself. Had been using an Arkel grocery bag to haul work-stuff in (which honestly, isn't much). I have a nice Geigerrig hydration pack that I thought I would try out. I like having access to water like that. I find I drink more frequently this way. Also, I like how the bike handles.

The "pinch" that got me to consider changing though, was I didn't like carrying around my pannier whenever I stopped to do an errand. I have a Wald 582 attached to the bike that I can throw stuff in when I need to. If I do an actual grocery run, I'll probably use all three.

How about a plastic tub?
I've been using this for more than a year and it works great, though maybe a little kooky looking. Once in awhile I use a backpack if I have the tub off for some reason or if I do my commute on one of my other bikes and don't want to bother putting a rack on. Usually if I wear a backpack (unless it is really light) I end up wishing I had the rack. I'll agree with the others that that bike handling and looks are better without the loaded rack, but to me it's not worth it. I hate the sweaty back and the weighted down uncomfortable feeling. I can always ride my unloaded road bike on the weekends if I want a sporty ride on a good looking bike. Perhaps it also has a lot to do with what each cyclist needs to carry. I usually have a heavy notebook and at least one or two heavy textbooks, +plus clothes, lunch, spare tube/tools etc., and it's awful carrying that on my back.

I have a timbuk2, that uses a 1" strap with a buckle on the main strap ,its on a loop around the 2" webbing..

and the bottom corner of the bag has a tri-glide to attach the 1" strap to either side .

I use a simple seat post mounted cargo rack. Similar to this with just a simple trunk bag strapped to it. I am able to leave shoes and some clothes at work, so I don't have to carry much back and forth on a regular basis. I use at Target bag to put my stuff in and then put it in the trunk bag. That makes it easier to grab and go when I get to work or back home.

Prior to having this I used a backpack. It was ok in the winter and cooler parts of fall and spring, but like others mentioned, it caused too much sweat in the summer.

I have used a backpack so far on my road bike. I don't mind it so much when my load is light and the air is cool. If I have to carry more stuff the weight is tiring on my back/shoulders. If its warm I hate the extra sweaty back. I do agree with the posters who said it feels great when you ride on the weekend without your backpack, but I would rather have this great feeling every ride.

My new commuter will have a rack, and I expect to use a pannier most of the time. When I decide to ride my old bike, it will be the backpack.

Grocery store pannier to put my backpack in, Trek Interchange bag for my lunch.

I found that a messenger bag / backpack in a rear basket is the easiest / most convenient to use...

This surprises me. What is that make the backpack or bag so indispensible?

I don't like backpacks. Mainly because I started developing back problems and I don't like the weight on my back. In addition I am getting too sweaty with a backpack. All our bikes have racks. Including the mountain bikes. So I am using paniers on my commuting and my mountain bike. And baskets on my old Gazelle.
The only exception: I have a small camelbak with a bladder only which I use very rarely.
But I guess it also depends how you grow up. Being from Germany I cannot remember that they ever sold bikes without racks back in the 70s except some street racing bikes and before mountain bikes came along. And even nowadays most commuting bikes are sold with racks and fenders over there.

I carry few personal belongings and I don't like to leave them outside when I lock my bike up.

I use a quick release pannier with a shoulder strap

This is okay as long as the bag is clean. When they're dirty (as a pannier near the wheels is wont to be) then you have to carry around something that gets you dirty as well. I can see why people don't like to do it.

I've done little commuting by bike, but when I carry stuff I usually use a messenger bag (I currently have a Timbuk2 classic), which for me works well for light to medium loads. I hope to do some commuting to one of my jobs this summer, we'll see how well the bag works then. I've never been fond of riding with loaded panniers (the few times I've done so), although like so many things it's probably something you just need to get used to. If I did more utility cycling I'd probably look more into these things. As it stands the bag and maybe a rack are all I really need.

As I said I generally use a pannier, but if I carry my laptop, I use my light briefcase as if it were a messenger bag, as I think it gets less jolts that way.

I like my Timbuk 2 classic large messenger bag for commuting (and general riding, day tours, etc.) I suppose I have more of an "aggressive" riding posture, and it fits nice and flat on my back while riding. I recently pinned a big flashing slow vehicle triangle to it for night riding. I've commuted in some pretty nasty stuff, and haven't lost a laptop...yet!

It's been a good bag for light hauling as well, but I've found a trailer to be much better for heavy/bulky loads.

You hauling textbooks? , lap top ? how much stuff? archer Q?

I carry a fair bit in my bag but very rarely carry a laptop anymore. I sometimes carry my iPad, but don't even have to carry that most of the time.

i make extensive use of a software product called Evernote, on my iPad, my phone, my laptop at work, my laptop at home, etc.

i upload documents to it, I upload emails to it for the projects I am working on and this way I always have relevant material with me, and handy to access. So even if I feel the need to have access at home at night, I access through Evernote, re sync and I'm done.

i carry enough stuff without carrying that too.

You can have Chrome make a bag however you like. My old roommate bought a used bag that was left shoulder so he got used to that and always had to have them make a new bag when he wanted a new one.

I never cared for mess bags, I like double strap backpacks. I have the MW Fitzroy(?) and it's been able to carry every random thing I've needed to carry home so far(a full bike/Grill for my pick up/24 pack). When it's really warm out, I might run a front basket but just stuff the bag in the basket
, though I hardly ever notice the bag unless I'm carrying something over 20~ lbs.

It depends on my commute.

20 miles one way to work- trunk bag
5 miles to a train- backpack

I use my backpack when commuting to work (20 miles round trip) everyday. My back gets sweaty but I bring a change of clothes anyway so I don't mind.

Commuted 90% of last summer with a backpack. Hated it! Sweaty, sticky, sore, awful. Never again.

I bought a rack and a large rack bag (and later, a smaller rack bag for when I need to carry less) and I love, love, LOVE the rack and a bag. My bags aren't side-mounted panniers but on-top bags.

I don't think this right. At least not unless you can go into one of their "custom" locations.

The Metropolis ( the largest MBag ) is sold as a "right-handed" bag which of course means it carries over the left shoulder (?!) and for those of us that would be ordering custom from the website - that bag is not part of the custom program.

The good news is that both Mission Workshop and Timbuk2 make bags that can switch right to left and I use the T2 bag when I use the messenger bag as it is a bit smaller than the Metropolis.

I wanted to like the Chrome bag, but the shoulder choice killed it for me and I really do like the T2. If i get another big messenger I will just buy the Mission Workshop one.

My bad, we have a chrome store here and I always forget they only have a couple retail locations.