From my experience over 3rd and 4th year, most OBGYNs I know seem very happy. But because they are happy doesn't mean someone else will be. Happy is such a subjective term. I would say shadow an Ob or pay attention on your rotations. I know, for me personally, I noticed them all enjoying what they were doing, but that was because it involved staying busy, ready to think and act at a moments notice and their personality fit with that demand, thus they were happy.
Hope that helps, even though I'm basically saying, only you will know if Ob's are happy.