I can tell you about mine and hopefully you'll avoid the problems I have.
Mine has a detachable base and doubles as a backpack with 3 outside pockets. The main cargo area has the clear holder and is the size of a tri-fold/clipboard.
So the problem is the zipper. It has the classic sized zipper and it didn't last one season. I tried to have it repaired, but that repair didn't last and it can't be replaced because it's glued and sewn together. So I added those belt clip things on straps to hold it closed, basically backpack straps that cover the lid to keep it closed. Looks cheap, but works.
It's not expandable so if you carry anything extra, you have to find a different place.
The problem is that I'm always in and out of the bag, having a standard sized zipper just doesn't cut it, you need something like a jacket zipper.
I can say that having it the size of a clipboard is just about right. I put my iPod, phone, gloves, 1L water, sun glasses and tri-fold (portable 8x11 holder), tools, etc... in there.
I can go to a fast food and put the bag in there and ride off.
It's a two part system, the base can stay on the bike and you unzip to a back pack but I've never done that. With a broken zipper, it doesn't work that way, but it is cool that I could with a fixed zipper.
I could put an "over flap" with velcro on it and then leave the base attached to the bike and have a back pack if I wanted to.
The base can be cable-locked to the bike.
The zipper it back on isn't so bad.
Overall, it's served me about 8 years now and I still use it.
This is what it looks like, hard to see the 2 parts, but the base unzips and it has the shoulder straps to make it a back pack.
If I were to do it over again, I'd get the expanding version like this:
Car001 Cargo Endurance Tankbag - Cargo Luggage - Collection
It looks like the same thing, but expands. I have the expanding saddle bags and you just unzip them, they double in size.
I think the zipper problem could be overcome with a velcro flap attached around the top. I went with "over straps" and quick clips, they don't keep the small stuff in there like velcro would.
The size is just right for a few books and clipboard, iPad, phones, 1L water...