We've been using Accelo for maybe 6 months. We're not fully implemented, but are coming from WORK[etc] and its a big step up. VERY easy to get going on, and lots of room to grow in to the product. We will probably be implementing forever as they seem to always be adding new features, but that is a good thing in our eyes.
I love the feature set and integrations. Being able to work with JIRA on our product side directly, as well as Zapier for more creative integration, is absolutely huge.
The biggest downside to this, or any product like it, is that you don't know what you don't know until its too late. Learning enterprise size products like this is tough, and you have to work many hours to figure everything out and get it fine tuned, but that is with any product.
Time used: Less than 6 months
Frequency of use: Daily
Likelihood to recommend