It's hard to point to just one thing that makes Teamwork so great, but the fact that Teamwork makes everything just so intuitive and easy to do is probably the best part. You can turn on and off features for different projects so they don't distract you from what you're trying to achieve. It's easy to get other people on the team involved and they don't need to ask a lot of questions in order to be productive.
You get recurring tasks, subtasks, due dates, isolation of users between projects and a whole host of other features right out of the box. Not every project is the same and Teamwork clearly recognizes that.
There's even a Chrome plugin that can be used to send data (like links, messages, tasks, etc) into Teamwork based on webpages you visit or information you come across that could be helpful for a project.
There's very little to dislike about Teamwork.
The biggest downside is more of a data management issue than anything else, but you do have to be a little careful about using it as a dumping ground for everything. It's very easy to just start adding things like links, notes, notebooks, etc, and then forget that they're there or lose track of them. The search capabilities are really good, but if you forgot something was there, you might have a hard time tracking it down.
Tags appear to be global in a way that makes them visible across all projects and all project groupings. Not the end of the world, but you want to be consistent about naming them or making sure you don't put something like 'problem-customer' in there, which might be seen by people who could get offended, especially if it was on something they requested.
Honestly, I'm hunting for things to complain about. I've used a ton of different tools like this and Teamwork is by far the best one I've found.
Response from Teamwork.com
Thank you for your review.
Great to hear all your pros, especially your use of the Chrome plugin.
Your best practice and advice on data management and being organized is very useful.
Check out our powerful new filter options, the following blog post will explain it in detail https://blog.teamwork.com/powerful-new-filter-options-now-available-teamwork-projects/.