no1mad 

I know that a few of you use these things. I've got a couple of questions for you-

1. What's your posture on the bike, and does that strap cut into you?
2. About how big is the belt? Could someone with a 38/40 waist (U.S.) find one with a big enough belt?
3. Does it stay in place if you load it down? Full water bottles, flat repair kit, and the other odds and ends?

The only experience I have with them is from about 20 years ago. I would cinch it down, but once the H2O went in, the bag would try to fall off.

knucklesandwich

Mountainsmith (Day or Tour)- nearly 4 yrs commuting, no complaints.
nuff said.

oldranger

I usually wear a waist pack, filling them with the items that often go in a handlebar bag. Typically riding in a "semi-aero" position, the pack stays in place better on a bike than while hiking. No problems with the waist strap. The straps on mine could easily accommodate 40" waists.

I usually place my water on the bike, but when necessary, a 27 oz bottle bottle goes on the pack just fine.

One of the advantages of a waist pack is that critical items (passport, money et al.) remain on my person when I am off the bike.

TiberiusBTkirk

38-40 no problem fitting.
I bought 3 waist packs, I've found a place near me that sells them for under $20. actually,
the most expensive one was $18.
if you're carrying heavy items like bottles that may shift and sag (like you posted) find one with a wide rigid back belt. like a weightlifters belt.
they should have side cinch straps to keep the stuff close and compact.
since I've spent money on 3 packs, they're used for different purposes. small for just one bottle
camera, mp3. medium and large for more than one bottle, cable lock, lunch, spare batteries and misc
sports stuff.

CCrew

They actually make wider ones with bottle holders for hikers now. I got mine at Gander Mountain for like $19, but Cabelas, REI, etc all should have them.

chipcom 

+1 - with the shoulder harness - and I have a 39" waist

I also have a Banjo Brothers Commuter pack which has a larger capacity and is waterproof.

Praxis

I had two Mountainsmith lumbar packs. They're great packs--they last forever--, but weren't comfortable for me while riding. It's either be too loose or too tight. Too loose and the back would hang down and catch on the seat at intersections and the like when I'm getting on and off Also tended to bounce around no matter what I did with the compression. Too tight and it was just uncomfortable around my midsection while biking.

I did like having my shoulders unencumbered, but eventually moved to a backpack, then a messenger bag. Like I said, though--they're well designed.

drjava

I've been using an old lumbar pack for about 4 years. I just replaced it with a North Face Mountain Bike pack which wraps around a bit more, but doesn't stick out as much. Still undecided if it will work out as well as the last. I think it will be better.

I ride a drop bar fixed gear and spend about 70% of the time on the drops. It doesn't dig in and cinched up right everything stays solid. I have the belt around my hip bones, just about where my belt loops are or a bit lower.

I carry my lunch, a paperback, mp3 player, spare batteries for my lights, tire repair stuff, and maybe an extra top layer.

This works for me since I don't carry much and my one way is under 10 miles. If I had to carry more, I would probably try a rack and panniers.

no1mad 

I've got the Large BB pack. Since my classes are mostly virtual now, it's a bit overkill. My main beef about it is that it''s all vertical. I have to take whatever is on top to get to what's at the bottom and then reload. But I bought because of the waterproofness (had a softcover text get wet in my old backpack). It will see more duty during the cold morning/warm afternoon months.