My opinion is, it's really its own game and not one I'd base on the others... H3 is pretty distinct in gameplay overall, and handles a little different than most FPS's in general, but that's not to say it's superior. I think it drops a bit of realism that other FPS's are trying to acheive... If that makes any sense.
I try almost any new game on Normal and see how it goes... Most let you alter it to easy if you have trouble, but I like the challenge at the start, and if I'm feeling saucy I try the harder difficulties, but some of them are virtually impossible to me.
I love playing Halo3 with the kids, but I swear children own the game top to bottom... They're just faster at it. Snipers especially. I can snipe, and with adults I think I'm ok, but against kids, kids just kick your ass 24/7 to the point they'll get bored with you. It's really sad.