I wouldn't get it if you want to play seriously and competitively. Halo 3 has issues with hit registration (sometimes ... seems like the bullet spread is too much, I don't know how to explain it, the shots don't hit the same like on the 360 version a lot of the time), the ranking is obnoxious, too many of the playlists are cross-game, so even if you like Halo 3, you never get to play CE when you get a hankering for it. And CE & H2 are not the Xbox versions, they're ports of the PC versions.
There are a lot, and I mean A LOT, of downsides to this game.
However, it is four Halo games (five if you get ODST for $5), every map ever (even the PC maps for CE & H2 and all the DLC maps for all games), forge modes for H3, H4, and H2A, multiplayer for CE, H2, H2A, H3, & H4, 4 player splitscreen on all titles, all 1080p (except H2A has a slightly lower resolution because apparently the switching graphics & sound feature was a lot to process and run at full 1080p), all 60fps. It's one of those things where if it was just the campaigns remastered and no multiplayer, I'd gladly pay $60 for it. But because of all the issues and apparent inability to completely fix after over two years even though they said they would, it definitely cheapens the game. I think it was just too ambitious of a project to do while also working on Halo 5 and they got in over their heads while trying to rush out the door on Halo 2's 10 year anniversary.
It's great fun to hop in the multiplayer and play some Halo (especially BTB), even the changes made to Halo 4's Slayer are great since they removed loadouts and armor abilities, but if you're looking to relive anything or put your competitive britches back on and start a new career of ranking up and kicking butt, you're in for a headache ... or serious challenge if you have serious patience.
It's definitely a must have if it's cheap ... you can fire it up and get a match in Halo 2 and Halo 3, just don't go in with your rose-tinted glasses on.