Our three man team recently took advantage of the FogBugz YC offer and gave it a try. There were some things we liked about it, but in the end it felt too heavy for us. It was more of a support ticketing system than an easy way for a small team to track their bugs (although it would probably be good if you had a lot of external users reporting bugs).
The other problem was that even a lot of custom tweaking, we never quite got the email subsystem working the way we wanted. We finally started using a plugin to make sure we all got emailed on every change, but then we got swamped because we couldn't figure out how to make it send only one email if you changed multiple fields on a ticket. It always wanted to send one for each piece of data that changed.
Before that, we'd been using a Google Docs spreadsheet, doing pseudo-joel-spreadsheet for project tracking and keeping a simple list of bugs.
Now we're using Wave and treating it as a digital whiteboard. It's got its share of bugs still, but I've been really happy.
We've got one wave where we do project tracking (including moving little name markers around, so we can all see real time what everyone is working on. And we've got another wave for bugs with same rules.
We don't put nearly as much info in our wave as we used to put in the spreadsheet (we did a lot of effort estimation and completion time tracking), but the simplicity may actually be helping us keep on top of it better.
And it's awesome to collaboratively rework our goals for each release and at any time we can just scroll through our bug list.