A lot of stuff has been written about Entity Framework versus NHibernate. Although with the release of Entity Framework 4, the gap became a lot smaller, NHibernate 3 brings the competition to the next level (at least that is what I think).
Dino Esposito did a blog series where he tries to compare Entity Framework(4) and NHibernate(2.1) on multiple areas. I don’t agree with his conclusions on all levels, but it’s still an interesting read: