It's really not a good idea to pick up an older Halo (since the server population is so low on them), so if you really want a Halo game, H4 is the way to go. I got the game for 40 during a sale right before Christmas, and I still play it regularly, so yeah, it's probably worth 40. The pros and cons of the game are as follows:
PROS -The maps (for the most part) -Playlists (they've improved a lot) -Commendation system -The Forge mode -Customization (armor, loadouts, etc.) -Weapon variety
-Poorly integrated ordnance system -Loadouts can be exploited for an advantage -Ridiculous hit detection (the DMR never misses a shot) -Infinity Slayer sucks -No visible competitive skill rank -Custom game options are limited
That's all I can think of. There's probably more cons that I've forgotten.
you can also put -No visible competitive skill rank as a pro as we wont have 100s of bots in multiplayer trying to ruin it for the real people