I've been in a number of PbPs. I have run 5 in total, 4 are still going. 1 failed due to player attrition and lack of posting. Of the four that are going, 2 are solo games. Of the 2 that are not solo games, I have had anywhere between 2-4 consistant players but have been able to keep the game going and only re-recruiting once. Those 2 PbPs started August '05 are are still going strong.
If you'd like to see them, I lift them up (and especially my players) as good quality examples. Go to the Playing the Game Forum and look for the Clutches of Evil thread or the Mightier than the Sword thread. If you want an example of the solo game that I am running, look for Into the Moral Darkness or The Sudden Approach of Unexplainable Power. I only ask that you do not post in the actual game thread. If you want to ask questions, you may post in the corresponding OOC threads in the Talking the Talk forum.
As for PbPs ... I like the slower pace. My player's characters are much more developed in the PbPs than they ever are in a FtF game I have been in ... but that is because the games take a long time to play. To do a PbP game you have to be committed to it, but I think the game itself is usually a more rich environment if the DM wants to focus on world building. [This isn't meant as a slam against FtF games, just that you can think alot before actually posting in a PbP game.]
I enjoy them thoroughly, but I am also a patient storyteller type. If that describes you, then you might enjoy it, too.
If you are interested in doing a solo game, do some research on whether PbP looks like it would interest you and drop me a line (if you do research on me and like my DMing style, of course). I can squeeze in another solo game if you'd be interested in that type of thing. But definately do some research first, paying close attention to posting dates (to get an idea of the speed of PbP gaming). Also, I would recommend finding a few games and reading a page somewhere in the middle of the thread (not the beginning). That'll give you an idea of the varying different DM styles. The great thing about PbP is that there's no right way to do it. Just find a DM style that works for you and you're good to go.