I wish I could give advice regarding the OMG certifications - I haven't tried any of them (and would admit a lack of desire to do so ).
I have, however, taken IBM's 486. I thought the recommended sources were fairly good and the exam was a fair representation of the information they sought to cover on their listing of topics. It wasn't too hard (iirc, I think that's actually the exam I've done best on, but that's likely out of my having overprepared for once), but I'll admit to using almost all of the time offered.
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!