Compared to truly competitive commander decks - where games usually end around turn 5 - it is pretty slow, but for a more typical metagame, there is enough creature-based ramp, which is all recurrable with Meren, that you can really get going pretty good most of the time by turn 5 or so, and once you get some of the sac-and-recurssion engines going with something like Grave Pact, things get bad for most opponents pretty quickly.  If you are always finding yourself getting taken down by tournament-greade Commander decks (something I personally find an abomination), you might want to look for a more social-minded group to play this format with.  In most more typical Commander groups, Meren is a pretty good, but not overpowering, commander.   Good luck!

It does pretty well.  I don't play in a hyper-competitive, combo-focused meta, more of a typical Commander/EDH environment, and it does well.  It has lots of synergy and if Meren is left alone for any significant amount of time, she becomes a huge value engine.