Halo 2 is a decent first person shooter but is way to overrated
Lets start off with gameplay, just like any other FPS simple same old guns, you got a sniper, rocket launcher, and an automatic gun (no big whoop).
The graphics are very nice no slowdons in the campaign but personeally I feel that this game could look alot better.
The sound is also done well, very cool music during scenes and sound effects of course are pretty good.
The value can depend on who you are, if you have an x-box live subscription this game MAY last you quite a long time (For me even the XBL got boring extremly quickly)
All in all I'd have to say that there are better FPS's out there that offter better and much more unique gameplay (such as F.E.A.R. or Half Life 2 or Unreal Tournament 2004)