Reading over the other discussions here will demonstrate that I wasn't impressed by Halo 3 to say the least. It also appears that more recently, there has been a growing community of people who are likewise underwhelmed.
As such, can I suggest an alternative for the FPS fan that demands more!! - Call of Duty 4.
If war games aren't your thing, then leave now. COD4 offers a fantastic single player experience. I really felt like I was Steven Seagal playing this (make of that what you will....!). The sensation of playing an active part in a big-budget blockbuster was intense. You can live out all of those 'black hawk down' moments without looking around to see if the girlfriend has caught you making gunshot sound effects whilst running around the garden! It is a little short, but playing on veteran for the achievements changes that.
Multiplayer is really something else, and shows how limited Halo 3 is. There is a massive amount of game types (16, I think) that offer great diversity. The weapons are meaty and great fun. Perks add a whole new dimension to multiplay experience. Progess through the ranks also unlocks new equipment, and the challenges based around the weapons and game type offer great incentive to play. it says a lot when I'm addicted to a game because of it's in-game challenges, rather than a multiplayer that offers gamerscore achievements, such as Halo 3.
In my opinion, it's hard to see how anyone could be let-down by COD4. Still not convinced, then just rent it first. You're garanteed to at least get some achievements out of it.
A side note: With relation to the whole review situation when Halo 3 was first released (the fact that reviews were not posted until much later after release, and even then negative reviews seemed to take longer). Does one detect a similar sort of situation to the one involving Jeff Gerstmann and the supposed reason for his unexpected firing from Gamespot? i.e. that other publishers have paid to keep negative publicity hushed up during the launch period. The situation with delayed reviews is common accross high profile game releases these days, yet reviews of other types of product are posted almost instantly..... Paranoia? you decide...