Welcome to the jungle ... Vietcong plus Fist Alpha combined provide many hours of tense replayable FPS action.
Purple Haze is "simply" the compilation of Vietcong and its Fist Alpha expansion plus the requisite additional MP maps for those so inclined.
What I would recommend is to play the Fist Alpha single player campaign before that of Vietcong. This would then follow the chronological sequence of the key characters and the development of the Nui Pek Special Forces base and its relationship with the nearby hill tribe villages.
As I only play PC games I will have to rely on the comments of GameSpot critics and others users about why the XBOX and PS2 versions of this great PC games is so bad ... it's a pity and perhaps some of that negative vibe has passed to the related PC games (VC, VCFA and VCPH).
Rather than rehash my earlier thoughts I refer you to my separate (and perhaps overlong) Vietcong and Fist Alpha reviews. But in essence Purple Haze is a great Vietnam FPS and in the same league as Men of Valor, its closest rival (to date), and both games/systems have their relative strengths and weaknesses discussed in the relevant reviews.
As a package, Purple Haze (plus the free Red Dawn MOD from Pteredon) provide great single player campaigns and numerous "quick fights" as well as incredibly detailed (multiplayer and campaign) maps that together makes the Purple Haze edition great value for money.
RECOMMENDATION: Simple, if you don't already have Vietcong itself then buy the Vietcong Purple Haze edition ... you WON'T be disappointed!
UPDATE: A brief review of the two major Vietcong MODs, Red Dawn and Rising Sun, can be found in my 23 Aug 2009 blog.