A better metric is to simply look at this forum as opposed to looking at the number of ROMS or dev activity. Not much going in as opposed to forums for other popular phones at this point in their lives. That's actually a good thing. People have discovered an unusually stable and capable phone and haven't needed nearly as much forum help as other phones. Not much moaning about what the phone can and cannot do. And, for me, I haven't felt the need to root the phone as I had for the Eris and the Droid X. Part of this is that I want to wait for ICS before doing it, but mostly it's because it feels right out of the box. I might not actually ever root the damn thing, which would be a first for me for the last three phones.
This is purely subjective but I spent a lot of forum time with the Eris and the Droid X researching and asking about "issues".
So, from a "success" standpoint, I'd say it's been successful. From a sales standpoint, I dunno, but I suspect that it's been successful, especially if you include the MAXX. The only real issue with the phone was addressed by the MAXX. And, for people like me who have the UrRazr, an external battery takes care of it as well.