I was an active, very satisfied Billmonk user with 9 or 10 friends. It was aggravating when Billmonk started choking; worse still when the degradation made the app more or less unusable.
If you're in the same boat, we've since moved to http://www.splitmybill.ie -- a service that has more or less duplicated Billmonk's use cases. Very basic, but meets our needs.