Large user base. Great for tracking defects but that's not really all it can do. It is used for tracking "issues" in general. Issues can include Defects, Change Requests, Requirements, User Stories (Agile), Sprint Planning (Agile), Progress Tracking, etc. The pricing is also excellent and they offer flexible subscription plans. Great technical support being a commercial product. Quite stable as hardly seen any downtime in the few years I've used it in different companies.
Room for Improvement:
The workflow and layout can be confusing and takes a while to get used to.