I'm on the free plan, all the basic options to run a project are there and the mail notification/comment system works very well. The user interface is good. My only complaint is that you cannot have more than one person assigned to a task. It's their philosophy and they say that if you want to have more assignees you should either duplicate the task or create subtask (only for paid plans).
Do you know an article comparing Backlog to other products?
You can submit it here