If ORMs were cars ...
Then Linq to SQL would be the Smart ForTwo. A seriously fun and easy way to get around town as long as you don't want to carry more than two passengers and maybe a suitcase.
NHibernate would be the HMMWV. Works great out-of-the-box if you are willing to give up some creature comforts. There is just no way you would drive this around town, but it can go pretty much everywhere. What's more, you can adapt it to a thousand different configurations with the proper kit. What's more, it has a passionate following of HMMWV afficionados that can help you get over any problems you may have.
Be careful though. You had better know what you are doing before you take one of these out for a ride ...
Finally, Entity Framework v1 is the Ford F450. In the body-and-chassis version.
Creature comforts, ease of driving, but you have to add some parts yourself if you want to use it for work. The good thing is that you can certainly do that. It's just that, unlike the Ford F450, there aren't any pre-built packages available for EF right now. Luckily, EF v2 promises not only to provide pre-built packages but it will also come in several pre-built models. That will certainly ease some of the adoption pain.
UPDATE: Ayende thinks that NHibernate would actually be this ...
The Hummer H2 Limo.