I've been following the O/R mapping scene for a while but mainly only as an observer. But for the past few weeks, I've been getting my hands dirty with WilsonORMapper (demo version) and NHibernate. Hands down, WilsonORMapper has a shorter learning curve and I also like the fact that the APIs resemble the APIs in ObjectSpaces plus support for most databases out there. The framework is well documented but it does cost $50 to subscribe to it. NHibernate on the other hand is a heavyweight and the mapping options that are available can be overwhelming.
Nevertheless, I sense there will be a strong following for NHibernate in the months to come due to the Java Hibernate origin. At version 0.7 beta, NHibernate supports many databases but lack a SQL dialect for MS Access. Anyway, I've made it work for simple query scenarios using generic SQL dialect but multiple joins posed problems. MS Access support for me is important since I do have projects running on low-cost shared hosting.