To be quite honest, I don't find it useful at all. For the past several minutes I've been trying to come up with a use case for it, but can't because I don't find myself needing a stopwatch whilst using my computer, unless you're in a research facility. Maybe if you can automate the stop watch and start/stop it according to certain actions, along the lines of RescueTime, then it could be a bit more useful. As far as premium plan, you have quite a ways to go to convince me to pay for your service. My iPhone stopwatch is sufficient for my needs and I don't see a reason to use your service.

Maybe you were going for a specific use case that I'm not aware of, and if that's the case, let me know.