You have so many options when it comes to choosing a ORM these days. My personal favorite is nHibernate I have been using nHibernate for a few years now and I find it flexible really easy to use and has a growing community of users. Saying that I'm always on the lookout for something new which is where OpenAcess ORM form Telerik comes in, I use the Telerik RadControls for ASP.NET AJAX on some projects.I have always found it a good practice to minimize the number different 3rd party control vendors in an application. You usually have less problems integrating the controls and have a more consistent product.
I have a simple project that does currency conversion currently the data access is implemented using nHibernate mappings. I though this would be a good place to start comparing the products. So with the intention of swapping out the data access code to OpenAccess ORM I took a copy of my project and removed the nHibernate code. It took me 5 minutes until I ran into my first problem the first object I had mapped was a view without a primary key.
OpenAccess ORM doesn't yet support views, this is not so important if you are creating a project from scratch with a new database design, but this is a bit of a show stopper for an ORM product if you are trying to map an existing database.
So for the moment my vote goes to nHibernate.