I've been watching Fogbugz for a while. Our previous solution was Trac + Subversion for about 5 years and it served us very well. Before Fogbugz added Kiln there wasn't enough of an advantage to making a switch. The other factor that won me over was EBS.
We transitioned to a self-hosted FogBugz+Kiln install a couple months ago and I've been very happy with it, especially with the development velocity and very flawless upgrade process and customer support.
If anyone has any questions about Kiln or FogBugz, I'd be happy to answer them here. I'm glad to see so many positive comments though, especially about Kiln.