Let me give you some feedback from a project that used Toplink.
Toplink was generally pretty good at doing O/R mapping in general and was easy to get started. We had some learning hurdles concerning the ExpressionBuilder syntax, but we got over that.
One issue that we did have was the Mapping Workbench. The mapping workbench is very fragile. If any of its generated project files get out of whack you can't rebuild the project - it probably won't even load. The MWB is so fragile that it won't even attmept to re-import a working project. It's a very dubious little tool. The only good thing is that the MWB project files are all plain text, so you can fix them up once they become hosed. There are also some settings in Toplink that cannot be set by the MWB. You will have to manually update the generated mapping file(XML or .java).
Right before our Toplink guy left, he, myself, and another developer had to spend about two days to get the MWB back together and checked into PVCS. BTW Toplink doesn't seem to like *any* version control.
I guess the upshot is that the technology is solid, but the MWB isn't.