Very simple interface, great control over time intervals. Projects, tags, clients provide enough depth to be organized while the whole concept stays clear. Reminders for tasks that were left to run overnight is a really great feature. Autocomplete makes task creation really easy, and it not just completes the title, but the project and tags too.
The autocomplete feature is really nice but learns wrong terms too, and the list of learned expressions can not be managed.
A bigger selection of icons would be nice, an icon-font maybe, or a way to upload custom icons. I would really like to color tasks/projects which would be represented on the timeline, the background color of the project icon could be a great start, along with color variations or custom colors for the same icon.
I would probably use the desktop client, but there's only a Windows version, and I use a Mac. I would love to have this on my mobile. A native app would be the best, but there is no mobile friendly design at all, which is the biggest problem I found.
I recommend you taking full advantage of tags, projects and clients to organize the tasks, this way you can generate really useful reports.
I wanted to keep track of my day, and based on the results manage my time more carefully. While I make myself track the various tasks during the day I caught myself beeing more focused.