Ok, If you've played GTA III/Vice city for the PS2 you won't be missing all that much by not playing the Xbox version. Having said that these conversions of the series are simply amazing.
Graphics: Probably the most noticable change in the Games. The Character models look great with light source shadding and articulate fingers to boot. The vehicles now all have reflection mapping and the whole environment just looks sharper. Not to mention the draw distance which has been greatly improved.
Sound: Another aspect of the Pack that's note worthy. The same great tunes from the previous games only this time with the added option of custom soundtracks. Though I'll admit I rarely use them in Vice City since it already has great music.
Controls: The controls are mapped to the Xbox's controller very well in my opinion. Using the L and R triggers actually makes the driving seem alittle better. And the anolog's position on the Controller S seems ideal for these games.
Gameplay: Essentially it's the same games as found on the PS2. Meaning you'll run through various missions be it driving, shooting or a combination of both. The more you progress in the games, the more of the story you will unlock.
Overall: As stated before, if you've played the PS2 versions of these games then you really aren't missing that much. However if you haven't or only played them briefly, then you owe it to yourself to try the Double Pack. Believe me you will not be disappointed.