I too feel Metal Arms is "fun"...It's a great game and I enjoy playing it, especially with my kids in multiplayer mode. Haven't finished the solo mode yet. It's on my to-do-list.
Halo to me was intense. It was a very satisfying experience. A bit repetative at times and overwellming in others (library). It's one I'd play again (once I get through the other 5,000,000 games I just bought). Another fun one to play multiplayer on.
--- oh yeah! Now I'm a REGULAR! ---