I used Assembla both as a developer and customer creating tickets for other developers in my team.
For ticket creation, and for me it excels at this. It's searching/filtering features are excellent, it allows creation of custom fields which can be searched like any other field, and performance is great. This is a difficult thing to do, and it does it well, because often you want to capture information specific to your organisation.
I love the messaging feature, to communicate between our team, it is really useful.
Also great traceability/audtiing of all edits done which is essential.
Also their support is prompt and responsive, this is really important when I have found some minor bugs.
Review collected by and hosted on G2.com.