We haven't gone over the Epic Level Handbook yet, only MM1, MM3, ToB, FF, Libris Mortis, EPH, HoH and currently MM6 (compilation of online monsters). We will get to it when we get to it, but I believe, considering the overall quality of other monsters, that it will not be people's first choice.
If you want to play it, then ask your DM if around LA+2 (maybe +1 or even +0 but being conservative is always best) is good. There are two problematic things with the Mercane, it's that enormous SR (you're basically immune to spells of your level) and Telepathy (for mindsight). The rest is subpar, even with such great stats on an Outsider), and Plane Shift, as useful as it is, isn't that great as a 1/day, and it is also pretty hard to advance.
Edit: also, if the dragon does know where it is going when it planeswalk, then change my rating to Troacctid's W:+1, VY:+0, higher: -0