A very fun shooter with great multiplayer.
I bought Halo 3 for the online component, never having played Halo 1 or 2. I've been playing the campaign for achievements' sake and it's entertaining, if a little confusing to the "never played H1 or 2" crowd. But man is it short! I've only been playing the campaign (on normal) approximately 3 days, and I'm on the third to last level already. There have been some nice set pieces and action sequences, but all around it isn't the best story I've ever experienced in a game. The friendly AI is not that great, either. I have been killed by my own side more times than I can count. Getting splattered by a bunch of marines in a Warthog right after killing the last Wraith is an incredibly annoying experience. Also, I've had a number of issues where I get stuck in a wall or a bush or something and have to reload.
Multiplayer is where Halo 3 really shines. The varied, intense and generally awesome matches are, for the most part, highly enjoyable and fun. Despite the game's reputation, I have had little to no incidents of the misconduct (read: t-bagging) Halo is known for. The only thing missing is a "narrow down" option. I am sick and tired of continually getting the same maps. I've gotten the Narrows map probably 40 times, whereas I've gotten Sandtrap once. A "maps I want/don't want" feature would be welcomed in a future patch or ODST.
Forge is a fun diversion when I'm sick of shooting things, but gets too long and boring too quickly. Building the ultimate infection map or simply screwing around with explosives and gravity lifts is highly entertaining, but gets old fast. Also, there are some imperfections in the model. No "clear all" or "undo" buttons exists, making starting from scratch or replacing an accidentally deleted keystone to a tower very difficult.
I haven't spent much time with Theater mode, save a couple "that's awesome" clips and once to figure out what the heck happened when I shot a rocket randomly, ran away and a minute later got a double kill medal. I find it strange that even after it says the video is loaded, you still have to watch it normal speed to "unlock" the entire video, no fast forwarding allowed. Also, being unable to pause the action and pan during a recording is annoying. All the same, Theater is a very good aspect of Halo 3 and things like this should be in more games.
Overall, my only problems with Halo 3 are nitpicks, nothing big. Halo 3 is one of the best games I've played on 360 and I highly recommend it.