On our current project, we've amazingly used 6 different tracking tools (2 versions of PVCS), mostly commercial. Here's my opinion on the ones that we've used. I've listed them in order of my most favored to least.
Serena Teamtrack - We use a web client. The interface is intuitive. Performance will vary across installations, but comparing with our same data in each tool, this works the fastest. It also works in Firefox.
HP Quality Center - This is also web based, but it is IE only. On the upside, it's well organized, easy to use, and full-featured. It has reasonable performance for us as well.
It has an odd feature where there isn't a save button. It saves automatically for you. To force a save, you have to navigate to another ticket. Also when you first use it, it has to install so many DLLs that it is practically a thick client. That being the case, IE sometimes gets locked up (usually when trying to reinitialize a session after session expiration). Once locked up, you occasionally have to kill IE to regain control.
Bugzilla - I didn't use this as thoroughly as the other tools, so this isn't a fair comparison. We used it briefly for some internal tickets. I suppose the big upside is the (lack of) cost. IMO, I just didn't find the interface as nice and easy to use as the other tools. Its been awhile so I apologize for lack of specifics for why I'm relegating it below the others.
Siebel - There wasn't much to like about their defect tracking tool apart from that it is better than PVCS. The interface seems hokey. It's as if the Siebel interface has a set of user interface controls and it tries to force all square pegs into its round holes. Another downside is that it uses lengthy generated IDs so its hard to reference them or search by them. Along with that, the ticket IDs aren't sequenced.
Merant PVCS - We had separate databases and used both the web client and thick client. Its been awhile now, so the details are fading. I recall there were bugs in the tool and they weren't getting fixed, for instance reports couldn't display certain fields. Performance was bad. It took a long time to load. It was slow to navigate through tickets.