For years I carried two extra tubes in the back pocket of my jersey. Recently I decided that I wanted that pocket for other uses, and ordered a saddle bag from Silca. That one turned out to be difficult to use. I looked at Western Bikeworks catalog and found this one. When it arrived I found it very easy to attach to the bike, large enough to hold two tubes, either sized for my road bike (25MM) or my gravel bike (32MM). I could also fit two CO2 cartridges and a patch kit in the the bag. I quickly realized it was well worth it to have one for each bike and bought a second one. It fits snugly under my Specialized Power seat, has reflective material on the back (you can't have too much of that!), and has been very easy to use when I have gotten flats on the road and needed to get to my extra tubes or to put the damaged on inside. It has a excellent zipper system, it totally waterproof, and pretty unobtrusive when secured below the seat. It is exactly what I wanted in a saddle bag.
These bags are of very good quality and very functional. Easy to adjust to your racks and the ability to move foward or backward when mounting. I've used them on a couple of bike camping trips with no issues what so ever.
Better yet, Western Bike Works always has great sales and excellent customer service.
This top tube bag is very well made had very light. I bought a regular (small) size for use on shorter top tube dropbar bikes, so it doesn't get in the way too much when stopping and standing over the bike. It has enough room for a regular size iphone 6 (definitely not long enough for the PLUS ones), a wallet, and maybe two energy bars. The main issue I have is that the bottom strap interferes with the same brand frame bags I have... I have three Apidura Backcountry frame bags, one full frame in small size, two half frame ones in small and medium, and they ALL have the same issue... Maybe it won't have the same issue with the extended (long) version of the top tube bag, or if your frame bag allows you to move around the top straps. By the way, Apidura's Expedition line frame bags do allow different positions for the top straps, but I tend to go with the Backcountry line for the light weight.