I can't offer a meaningful answer to what you're asking - I haven't, myself, taken the OMG test, or seen candidates come in for interviews here listing it. I know that I found IBM 486 useful, but only in terms of confirming to myself that I understand UML and as a confidence-booster towards the SCEA exam.
But, I would put in something that might help your deciding - 486 isn't a stand-alone certification. IBM uses it as a component of developer-level certifications, in concert with other exams (you don't get a separate certificate for 486, but rather just your score report). Food for thought.
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!