The critics are being ultra harsh on this game. It is NOT a new game, it's a port!
The game's level designs are still better than most of the new 3D Sonic games to be honest, well... at least Sonic's levels anyway. They are fast, fun, and not too hard or too long to navigate through, with the occasional glitch of maybe falling through a wall or a floor, but it doesn't happen as much as the critics have been saying. There's enough checkpoints and lives in the game anyway for that problem to be refuted either way. The achievements are a bit of a mixed bag, since some are super easy and the others just take a long time to get, but hey, at least Sega added some achievements in for the game!
The graphics are of course outdated, but critics are making too much of a big deal over this. This game is like 12 years old for crying out loud! I'm not trying to bash anyone, but from the previews, it was clearly an untouched port of Sonic Adventure DX to begin with anyway, and having both the PC and Dreamcast versions, I will say that the game still holds up fairly well to be honest. Sure, there are low-res textures in places, but I could say the same about Ocarina Of Time or Super Mario 64! It's the gameplay that counts, not the graphics. The voice acting is cheesy and obviously sub-par in today's world, but this game still has a charm going for it in my opinion.
Levelwise in terms of other characters, newcomers will probably dislike Big The Cat's levels the most, due to the fact they are slow and tedious in parts of having to fish for creatures and his pet frog named Froggy, but it's still passable in my opinion, whilst Tails' levels are just Sonic's but shorter and more race-like, it's still good fun finding new shortcuts and flying around in my opinion. Knuckles' are about treasure hunting and exploring, but I think they're still pretty good. Amy's is passable but very short, whilst E-102's in my opinion is the 2nd best due to the shooting gameplay.
The game's replay value on Xbox Arcade varies on whether you got the DX addon or not. Vanilla Sonic Adventure will still take you a few days to finish, since you have 6 characters to finish, plus you have to collect all the emblems scattered around the game to finish it 100%. The DX addon adds missions and Metal Sonic, but to be honest, I think Sega should have just included this into the pack, and maybe the price tag would have been easier to swallow, but I'll end it with this statement:
If you have played Sonic Adventure in the past, you might as well rebuy the game since its only like a fiver anyway. Newcomers who have never played Sonic Adventure, be aware that this game is 12 years old and NOT a new game. If you keep your standards like that, you'll most likely at least enjoy the Sonic stages, which should be worthy enough of purchasing. So, for those reasons, I give Sonic Adventure a 7.0 out of 10, meaning after 12 years, the game is still good fun, but a little dated as expected.