I havn't used it, I researched a few options when we first started and WHMCS came out on top as far as support and active development went.
Modernbill has just been bought by Parallels, who have a habit of buying products just to kill them off, so I wouldn't want to go down that route now anyway
Another decent solution is ClientExec, i've been told their new version is great - Although I don't plan to move anytime soon. We've just sponsored a module for WHMCS which should make it into 3.7 in a few weeks which will make things alot easier for us.