I used Atlassian JIRA for bug and issue tracking at my last job. I absolutely loved it and it was particularly easy on the eyes.
My present company is using Trac instead, and while it does do all the basics, I am finding it really lacking, particularly with the inability to easily setup multiple projects and link issues.
Oh, and the fact that it uses SQLLite is a bit of an issue for me to.
Does anyone have any other good reasons to switch?
Answer
I have used Trac for several years already. First of all, the sqlite issue... You can easily switch to MySQL if You like to. We began to feel a bit of a laggy about the interface when the wiki table exceeded 1 million rows (a rouge script did that), so I guess You should be fine about performance with Trac. You can also use multiple nodes serving Trac, if that will be a problem (it's good to have 2 in case of a power/network failure anyway).
If Your team has only 6 coders, You could consider putting all projects in a single Trac project and using the ticket's component or milestone fields. It should work just fine.
It is also worth to note that Trac is extremely extensible through plugins that can be easily obtained from Web.
I have no Trac link issues whatsoever so I can't comment on this problem. Could You be more descriptive?
And about the inability to easily setup multiple projects... Our admins wrote a ~100 lines script that handles it and now it's like 5 minutes to set up a new Trac project. We have like 60 of them already. I don't know about Jira, but new projects are being set up rarely. You will want to create a report through a ticket query quite more often.
If You consider commercial solution, take a look at FogBugz - I've heard it's amazing... However it's not as extensible as Trac (You can't just use the rich python api to add a functionality You need - and I value this feature very high in Trac).
Take a look on a length of Trac plugin list. You can easily install any of them if You like to.
EDIT: I found out later that it is easy to add projects in Jira, but Jira is harder to grasp than Trac due to the number of features it supports (and you probably won't use most of them in a 6-man team). Nowadays though, you need to consider Jira because of the Agile plugin.