Shadow siege is a stable, well-developed roleplaying enforced mud. It shares some common features of RPIs: character advancement comes through roleplaying and the world is kept fairly small to concentrate the population. I really enjoy Shadow Siege and suggest giving it a shot. There are a few things that set it apart from other RPIs.

1) Dynamic world, imm-plots: The setting in Shadow Siege is always moving forward. New areas are discovered, governance changes, players are given an opportunity to advance their position, and more. Imms do participate in plots and player ideas, and you can easily get an idea of what's going on here: http://shadowiki.com/Wiki/index.php?title=Current_Events

2) Good crop of roleplayers: while the population is lower than I would like, the regulars are really a pleasure to rp with.

3) Solid original setting: interesting lore, with plenty of mysteries to be solved.

4) Conflict: there's an established tradition of good v evil conflict, rp is never just a big love-in where everyone sits around the campfire.

There are some drawbacks too, but on the balance, these are the things I look for in an RPI, and Shadow Siege has a nice mix of them. Hope to see you there.