To start off, this is a must-own for any Xbox owner. The stories are engaging and allow you to choose the pace at which you complete the main stories. That, along with the side missions, make these two of the most compelling games.
The graphics have been tweaked since the PS2 version, which is easily noticed. Small things like the reflection of light off of the car, or the fire that shoots out of the Cuban Hermes exhaust or the flame thrower, make the game that much better.
The addition of custom soundtracks is welcome. Nothing beats crusing down Liberty City or Vice City listening to Nirvana or Catch 22.
Driving will take awhile to get used to if you've played the PS2 version, gas with the right shoulder, reverse with the left. The targeting system is still buggy, but slightly improved.
All in all, great game(s) that have endless replay value and will entertain you for weeks, if not months.