If you ask me everyone needs to get off of the "All card games suck" bandwagon, grow up and form their own opinion about each game. Just because a game has cards doesn't mean the game instantly sucks, as is the case with this game, if you ask me. ("If you ask me," meaning my personal opinion -- hence the flamers can leave me be.) I've been cutting Chain of Memories down since I finished the game (which happened to be two days after it came out in the US), but not because it involves cards. In reality, the card-based battle system is probably the best feature the game has to offer, and if you're patient enough to sit down and learn how to play the game, is actually a nice change of pace from the first game, which despite what people say was actually just good old "attack!," "heal!," and repeat.
Anyway, I'll be the first one to admit that the game has problems, most notably are the Room Cards and doors that require cards with an exact color and number, and with most of the worlds being reused and completely uninspiried (notice how none of the worlds after Traverse Town involve the main story until you reach Destiny Islands and Twilight Town?). We've all had the problem of having to track down a randomly generated blue, red or green card with a certain number to get into a door to continue the game. There's also the issue that some enemies (Defenders, Neoshadows, Riku VI, Ansem) are overpowered/overly annoying, and sometimes the game just stacks things so far in favor of your enemy that the game gets far too frustrating. Even so, the main game -- especially the storyline of the main game -- was done nicely in general. It wasn't anything spectacular, but it was original, and it was something to help pass the time for fans waiting on the true sequel.
The biggest let down, if you ask me, was Reverse/Rebirth. It failed in nearly every way because there was little story, hardly any character interaction, no customization options, and limited healing options -- all of which lead to an utterly boring and even frustrating (again, Riku VI and Ansem) extension. The ability to see things from Riku and Mickey's perspective was a nice idea on paper, but the end result wasn't nearly as nice. Considering that Reverse/Rebirth could be completed in less than half the time it took to finish the main story, it wasn't much use in the "replay value" factor, either. Since I never even considered playing the versus mode, I can't really comment on that -- though I wouldn't have expected it to be very fun.