It's easy to add new features or change existing ones; it has great support for custom fields (e-mail notifications are particularly useful); there were no downsides that I can think of! We're using this as part of our bug tracking system at work - we've been very happy with how quickly they implemented changes from us when needed. The only downside was having too many different projects all in one place which made management difficult sometimes but overall their customer service team have helped resolve any issues really well so far. As mentioned above though, being able to manage multiple project bugs through 1 ticketing tool makes things much easier than if each had its own separate account.
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