Halo Reach is a well-made game (no doubt about that).
The complaints about games like Halo is that each new installment is basically the same as the last with new content, but if you ignore that and just look at the games individually they're each good games. I would get at least 3: ODST and Reach. Reach and 3 (just regular plain 3) are more similar (you're a spartan in both, but reach takes place right before Halo 1) but I like Reach's story (plus it has a much more vast multiplayer experience than Halo 3 by itself) so I recommend it.
In ODST you have to be more careful, you're not jumping into crowds as much as thinking of a strategy of how to get from Point A to Point B without getting MURDERED. You're a regular Orbital Drop Shock Trooper (not spartan) so you're not as powerful, can't dual wield, need med packs to restore health and stuff like that. I would at least get Reach and ODST and if you like it and want more get Halo 3. ODST has very chill music in some parts while 3 and Reach are more epic and dramatic. If you have money just get all three of them, you won't regret it. And if you're into RTS games then Halo Wars is awesome. Don't believe anyone who tells you its crap, they're butthurt that it's not a shooter like the other Halo games.