Currently hosting 6 (I think?) managed servers with NationalNet. They're doing a capable job, no major complaints. I'd love a bit more foresight and planning, but my side is as much at fault as NatNet is. Bill is a prick, but a very capable prick, so thanks for a job well done, Bill.
MojoHost is the second coming of Jesus to me (I also host there currently), so anything I say about them is definitely biased. The cliff notes are that I'd eat the corn out of Brad Mitchell's shit to stay with them. Just sayin'.
I don't think you can go wrong with either Mojo or NatNet. Sorry, but I don't have recent experience with ISPrime, so any comments would be ancient history by now ... and none of them would be negative, incidentally.