Sonic Adventure was Sonic's first 3D game, and like Mario 64, it amounted to a whole bunch of various crap they threw against a wall to see what stuck. Unlike Mario 64, though, most of it kinda sucked.
Adventure 2 is a streamlined version of Adventure 1 gameplay-wise. It took the multiple levels and whittled it down to the fun ones. So imagine the decent to mediocre Running, Shooting, and Looking For [dukar] levels from Adventure 2, and add in Running After Someone levels, Running Away From Something With A Hammer levels, and Fishing levels. Think of all the most annoying, boring, stupid-ass [dukar] in Adventure 2, and think to yourself "After two years of revision, this is the stuff they kept."
Still, there's some fun stuff in there, and if you don't care about the Super Sonic level (which is cool-looking but kinda boring and/or annoying to play), I'm pretty sure you can just play the pretty fun Sonic and Tails levels and ignore the boring Knuckles levels, annoying Gamma levels, stupid Amy levels, and stupid-ass Big levels.
If you hated Adventure 2 overall as a game, you'll really hate Adventure 1 overall. However, if you loved the Sonic/Shadow levels, and think it'd be worth five bucks to just play some more of those and ignore the rest of the game (or "game", as the case may be), go for it.
But on the other other hand, the good levels will probably be in Generations anyway.