You should try Sequel. It does things AR and DM don't (CPK+FK for example), is generally faster, and gives you the flexibility of working with Models or raw data-sets depending on what the situation calls for.
I've used and written a number of O/RMs and Sequel is definitely the best.
This is really helpful. I will definitely check it out.
Also, I'm really amused that apparently the creator of DataMapper is (a) selling me on using Sequel and (b) the testimonial on their front page. (Not that there's anything wrong with that.)
Sequel and DataMapper have different purposes I think. Sequel is a lot more transparent in the translation between the ORM and SQL and allows a more powerful control of SQL statements. DataMapper allows some very powerful controls on the ORM and Ruby syntax side.
(Now that I am in Ruby land though, I am a little sad that arel/ActiveRecord/DataMapper do not seem as on their game.)