I found the XLAB after tiring of other seat cages that ejected my bottles like depth charges when I hit dips going through intersections. Riding in Arizona, we can easily drink 60 ounces of water on a 30 mile ride in summer, and this is a great way to take an extra bottle, including insulated Camelbak Podium 21s, which fit. I like the tight fit to keep my extra bottle in place over bumps. Regular uninsulated bottles fit a little less snugly - but still pretty snug. Relatively slimmer insulated bottles like the Podium 21 is very tight, but work. I also have a Speedfil 40 oz. tank system for primary drinking while pedaling. I use the XLAB Aero to carry Heed electrolyte replacement to drink at stoplights, or on a shorter/hotter ride, just water to dump on my head. The seat bag part of this rig holds a surprising amount of stuff, although an extra tube is very tight if you have many other tools. The hardware is very sturdy and adjusts to give you a range of angle and depth options. Once on, it's solid as a rock. Nothing rattling or wobbling. Assembly seemed intuitive. If I had one gripe (let's call it a suggestion, more accurately), is that a tab or loop for a rear light would be a nice addition. For technical comparison, I have this on a 54" Cervelo S1 (aero seat post), with a Cobb Delta M saddle.
I was on the fence about getting the single bottle holder because it blocks (on my bike set up) the use of a traditional saddle bag and I don't like filling up my jersey pockets with all the 'just in case' tire repair stuff on top of nutrition that will be in there anyway. This has just enough room for the essentials - I have tire tools, a tube, 3 CO2 cartridges, a patch kit a multi tool and a CO2 dispenser head in it. You do need to be very deliberate about how things are oriented and placed in it to maximize what it will hold. It's not in the way and seems to be an aero design. It will definitely make the grip on the bottle in the holder quite snug. I left a full bottle in it over night to help stretch the bag a bit.
One thing nice about the tight grip is this will now keep a 25oz Camelbak style bottle in there without ejecting it on bumps, it wouldn't before - only bottles with a more traditional shape. Very happy with it, good purchase.
Super sleek looking pouch that fits the essentials for changing a tire. Hard to speak on the aero benefit, but it looks the part.
Small catch though, it makes the bottle significantly harder to get out and in. No issue if you only access the bottle once or twice a ride, but this would be a no-go in my opinion if it was your main drinking source. In my case, its not so I luck out.
As a previous reviewer mentioned, at first when adding this to a gorilla cage it is almost impossible to remove a bottle while riding. After leaving an oversized bottle in the cagepouch for 3 days, the holder has loosened and it is possible to remove a bottle while riding. As far as space, I have 1 16g CO2, 1 nozle, multitool kit, 2 tire levers, tube, valve core wrench, tubeliss tire patch kit, and chain quicklink in the bag. There is still a little space left, but not much.