Otherspace and Star Wars RoE are not the only MU**'s I've played, but they are definitely my favorite. And not just because they are free.
Otherspace is unique in that it's completely original. The races and story lines are all interesting, and it doesn't really matter WHO your character is, they can get involved. I've played a few different characters on Otherspace, and each of them have had a chance to get involved in some pretty important plots.
As for Star Wars RoE, I think the game is on its way to being really cool. Your character is free to roam the galaxy, and you can be a trader, an Imperial, a bounty hunter... basically anything you want. I think it's a great format for a Star Wars game. And since the rebellion will need to be run by the players, it's going to be interesting to watch things develope.
Both of the games strongest points are their player bases. I've had more fun RPing with the players on these games than in just about any other gaming experience I've had, and I've been gaming for a long time. The storylines are all entertaining, and the opportunities to really participate are great.
I think the weakest points of the game would have to be some of the judging that has gone on. There are a lot of gut calls that have to be made, and I think that there lies problems in this. Of course, for it to remain a MUSH and not a MUD, it HAS to be this way. But I think it's fairly easy for the administrators to pidgeon-hole particular players. I've played the games for over a year, and it was the administration that caused me to choose to leave. I felt like what was okay for other characters to do was not okay for my characters to do, and I felt that the rules were changed 'just to get me.' I do not believe that the administrators of the games are as a whole bad, but I think players just need to be aware that there will be situations that come up where they do not like the results. But I think the administrators sometimes push the game over the line of being a fun GAME to play.
However, even with its faults, I enjoyed all of my time RPing in the games up until I left. I'd recommend them to anyone who likes RPing online.