Is there a limitation to the number of users?
No, there is no limit to the number of users.
Is there a limitation to number of people, or the number of projects?
No, there is no limit other than the size of your disk storage space for your MySQL server. Currently MySQL can tolerate efficiently 2gig databases. The largest databases we have seen are in the Megs range.
What is the responsiveness of the tool?
Each database query has been optimized to reduce database joins or wasted data lookups. Special cache is built in the background for the most commonly viewed pages.