I have a frame bag and I have been using it on my bike that does not have a rear rack. For the spring rides I need to have enough space for my jacket and some colder temp gear. It has been working well even with the cross winds. It seems odd that the OP thinks that this is a relatively new idea, my frame bag is 30 years old. Maybe it just never caught on 30 years ago so they quit making them. mine was made my velocipac.

I wouldn't be deterred from a frame bag if I lived or commuted in an area without strong side winds. A frame bag could be pretty handy. But my area has some stiff winds at times and it's easier to remove a trunk bag or panniers if needed. A compact frame bag with quick releases might be okay in my area.

Sunday afternoon I wanted to test some new panniers -- empty -- on an east-west route with south winds at a steady 11 mph and gusts up to 30 mph. I didn't notice the steady wind causing any particular problems but a couple of gusts felt like someone had shoved the rear of the bike sideways.

I wouldn't mind a frame bag so my pump/tools/lock/tube could stay out of my panniers every day. I'm just looking for one that doesn't cost an arm and a leg for bikepackers.

I could get behind this aspect, smaller stuff that generally stays on the bike. I couldn't use one alone for my commute, I have to bring a full change of clothes, etc.

I am also having wild ideas about making one... but there simply isn't time for that stuff with riding weather coming up, bikes to fix, a bathroom to finish, and a baby on the way.

Everything but the lock can go in a small saddle bag, and then you can usually frame mount (bosses) the pump & lock. Frame bag actually seems best for clothes, so they can be stored flatter, hopefully getting less wrinkled than in panniers.

The seat bags interfere with seatpost light mounts. Yes, I could clip one on there, but it's not the same.

Ah, ok. My bike's all have 6-10in of seatpost, so there's no conflict. They do make small bags for the front triangle:
Something like:
Amazon.com : Deuter Front Triangle Bag bike frame bag black : Bike Pack Accessories : Sports & Outdoors

Big enough to hold everything but a solid U-lock, and shouldn't interfere with other items.

I just noticed that a few people took issue with my remark about trunk bags and panniers not fitting into the "Wow! That's functional!" category.

I guess I chose the wrong expression. Yes, of course my trunk bag/pannier set is very functional. I use it to carry everything I need for work, for overnight stays, and for emergency repairs. But it's still just a bag that gets strapped to a bike, and it even requires extra hardware just to mount it.

A frame bag, on the other hand, takes advantage of a wide open gap that's just SCREAMING to be converted to storage space. I think making use of this existing space is a lot more clever than adding extra gizmos (like trunk bags and panniers) to other parts of the bike. "Functional" may have been the wrong word.

They make 1/2 sized ones.

So just an arm OR a leg.

There are many, many inexpensive frame bags out there that would work fine for commuting. I've set aside my racks and panniers for a seat pack and frame bag. I just find that the bike handles better.

I have a buddy coming through with a Jandd one from his personal stash as soon as this evening. I will report back.

Revelate's 1/2 size bag looks nice for the money...$54 for the red, slate, or digicam on their site

Linkylinky!

Top quality stuff. In fact, I have many Revelate bags, but no limbs to actually ride any longer. It's a shame.

This made me laugh and Dr Pepper went up my nose. Well played

The Tangle leaves room for the bottles but getting access to the bottle while riding is a very different story.

Yup. Most of the bikepacking comes out of mountain biking and mountain bikers aren't nearly as resistant to Camelbaks as road riders are.

"Computer Numerical Control". You make the design on a computer using a computer aided design (CAD) program and feed it into a machine that converts the CAD drawing to physical object.

It depends on the usage. If I'm doing a road tour, I use panniers. It's just easier to pack stuff into them. But if I'm doing an off-road tour, the bikepacking bags make a lot more sense. The narrower profile means that the bike is easier to handle on rocky, narrow trails and jeep roads. The load is carried higher than a traditional touring load but while it does have an effect on handling, the other alternatives for off-road touring have their own problems. Trailers aren't any easier to use on rocky narrow roads and jeep trails. If you are going to use panniers...I have...you can't use lowriders for obvious reasons, so the handling is already impacted.

I bought a used (supposedly barely used) Revelate Ranger bag on ebay a few days ago. It's enroute ;-) - so my setup when needed is:
Handlebars: Revelate Sweetroll or Revelate Harness, Handlebar Buckets (waterbottles and stuff)
Frame: Revelate Ranger or my current Revelate Tangle
Seat: Revelate Viscacha
Optional: small backpack or stuff on top of Viscacha via bungees, also possible to attach top bag onto the Revelate Sweetroll or Harness

Xl porta-pack out of NH, one guy shop, $ 25.00. Fits all 3 of my Surlys and my Farley fat bike. So no. Fits bike pump, battery light, phone, glasses etc. Try Ibera or Kada as well.