If you enjoy games like Half Life 1/2 and System Shock -- run to Bioshock.  It is an amazing title.  The game combines awesome gameplay, good story and requires that you think to beat it.  The gameplay will not be the same for everyone since every situation can be handled in multiple ways.

This is Game of the Year so far, but Halo 3 will probably pass it.  I highly recommend that you get a copy of Bioshock -- if you truly enjoy gaming and want to see gaming moved forward -- get Bioshock.

If you just want more of the same and can't handle more than just shooting aliens, don't get this.  It is not a Halo clone (I love Halo, but this has a lot more depth than Halo so far).  Halo pushed forward excellent FPS gameplay and design, but not really the art of telling a story or allowing the player to completely experince a different game from another player.  This game isn't like having the needler vs having the rifle, it is completely different than that.  It also has multiple endings depending on your actions.

Bioshock(Aug), Halo 3(Sept.), the Valve Orange box(Oct.) and Call of Duty 4(Nov.) make for a hell of a fall on 360.