It seems like crap during the first time you play, but gets better as you progress.
Graphics: I've heard some people say that the roads constantly look as though they're covered in oil. That may be, but it looks much better than dry pavement, which looks drawn with no dimension. The tree movement is stiff, giving ie a bit of an unrealistic effect. The color scheme for some of the civilian cars are strange sometimes. The cars that you drive can look very good, depending on how well you customize it and the type of paint you use.
Performance: The game, or mine at least, doesn't have a bumpy framerate when a lot of things are happening. This is perhaps the only thing this game has that NFS: Most Wanted didn't. In Most Wanted, the cars were going so fast during a Level 5 pursuit, and I got caught once because of this, all over a mistake regarding the positioning of a cop and the tunnel entrance.
Map Layout: After I finished the opening sequence, I took a look at the map. I had just put the game in after playing Most Wanted, and instantly saw that the roads were 75% the same. Only the environment around the roads and composition of the roads were different. This can be an advantage. If you loved the layout of Most Wanted, then you'll love this one. If you wanted something different, sorry to disappoint. The highway system is very realistic, too. It's very open, has smooth turns and long runs, and has more trucks spawn here than anywhere else.
Car Selection and Customization: The car selection in Undercover is pretty much the same as in Carbon and in Most Wanted, with a few additions. The Camaro Concept and Zonda are some of the good additions. The PS2 version (sadly) lacks the Lancer Evo X. This car got me extremely far in Pro Street, and has very good performance. That, and it looks great.
The customization for the cars is very similar to the other 2 games as well. Just the organization of the vinyls is different. They are now based on company like Chicane, not type, like "Stripes" and "Flames". This may be frustrating to anyone that doesn't have the game guide. With it, you can browse through which one you think would look good on your ride. That way, you don't need to wait for the vinyls to load, only to not like the selection.
(Most Importantly): The 5-0 (the cops, for those that didn't know what it meant when they used the term in Most Wanted and Carbon) is very good in Undercover. They act just like they did in Most Wanted, with a few kinks here and there. The new vehicle layout is very impressive. The Civics and undercover cops are still the same, but now, they replaced heat level 3 cars (not sure what they are) with Mustang GTs, Federal Undercover cops with Porcshes, and Corvettes more Porcshes and SUVs again. The tactics are similar to the other games, but this time, just like in Most Wanted (unlike Carbon) the civilian cars are now active during a pursuit. For some reason, they were turned off in Carbon, but were hardly effected in Most Wanted. BUT, what they DON"T do in this that they did in Most Wanted was vanish off the road after 15 minutes. When that clock hit the higher times, the cars just went away, taking the realism from the pursuit. The fact that you can become the cop and earn money for it is very helpful. The last time something like this was done (be a cop and be rewarded) was with the Hot Pursuit series.