I was using mercurial in a project a while ago. I as a command line user was pretty happy with it, but our eclipse users where not. That may have changed, since my experience is not really up to date. I have also worked on a git project, but there we did not have eclipse users, so I don't know how the well the eclipse integration works.
There is now an eclipse plugin called eGit (http://www.eclipse.org/egit/)
Yes, back then there was also a mercurial plugin, which I don't think has gone now. It is not about the existence of such a plugin but about its quality. Maybe some eclipse users around here (and of course those using other IDEs) can test either plugin and report about the quality they experience.
This is why I mentioned that a major eclipse project (CDT) has adopted eGit. I think that's an indication of good quality. Regards, Nate