I bought the Topeak rack and DXP bag with Panniers also, used a backpack and seat post rack for years, and would never want to
change back. I can not commute to work,(work out of my house), but rather commute mostly for shopping.
I have a choice of 3 towns to ride to, closest, 12 mile round trip, very small town, 2nd 22 mile round trip, a few
more stores to choose from, 3rd 35 mile round trip, everything you could want there. It is a little cumbersome
once you have the panniers loaded, so I try to work it out that I do everything I want to like eating out etc...
with the panniers folded up, then when leaving town do the big stop and load up...

i've never tried panniers. maybe it's because as a kid my bike had huge baskets to carry newspapers that i delivered and I have bad memories of the weight of them shifting my balance. Also riding around with my friends who all had BMX bikes and there i was with these baskets clanking around the rear tire of my Ross 10 speed. *sigh*

a backpack carries my lunch, clothes and tools just fine thanks

I've tried the backpack. I have an average 1 hour 40 minute multi-mode commute each way and there is NO WAY I'm gonna put up with a backpack and those hassles that long.

I use 2 Topeak racks. A quick-release beam with side frames, and their standard Explorer, depending on the bike I'm on. With the Topeak's biggest trunk bag that has the built-in panniers (that zip/fold out of their built-in side pouches). They can easily hold laptop, complete set of clothes, lunch, and way more. And if you only need just one pannier, all the better (folded back into pouch -- no sail). I love that "slide-and-slam" motion that locks the bag to the rack. Guess I could gently latch it, but the "slam" is just fun. I do pull out the built shoulder strap at times - love how that attaches to the bag.

I'm at least 4 years into using that MTX bag / style and can't imagine using anything else unless I get into self contained touring. I'm a credit card tourist at this point.

My other rack/pannier rig is the Xootr CrossRack (built expressly for the Swift folder) and matching eleven-71 bag. Now that's a real sail, but it's amazingly versitle. It will easily hold a 12-pack and complete fixinis for a steak dinner for two. A laptop and clothes are not a problem.

I DO use a backpack with my bike that LIVES at the office. I'm happy using it for short jaunts to the grocery or hardware, on the days that I arrived by car. I'm very lucky to have indoor storage for a spare bike. That's a feature !! Some days I just arrive early and ride for an hour or so. Normally leave the backpack at the office for those rides.

Good luck getting settled in.

I wear a bag (I have several different kinds) if I will be on and off the bike running errands (other than huge grocery trips) that way all of my flat fix tools and such stay with me automatically. I use panniers for longer/larger hauls.

Often, I use both - sorta. I have a small shoulder bag that fits perfectly into on side of my panniers. It stays there with all of the stuff that I need every day inside of it. The other side of my panniers is for hauling lunch, clothes, small grocery runs etc. When I get to the grocery, I take my shoulder bag out of the pannier and wear it into the store. When I return to the bike with the groceries, I keep wearing the shoulder bag - now I have both sides of the panniers for groceries. Best system I have found yet.