Our clients do tend to use Qlockwork in a wide variety of different ways to track their time. We have some who even find a 2 minute timeslot a little long whilst others have asked us to introduce 30 minute slots.
On balance, more seem to like the smaller increments than the larger ones, particularly if they spend a lot of time handling email in Outlook, where they can be moving between emails very rapidly.
In the office here, we tend to use 5 minutes as we want to try to force ourselves to stay longer on each task, although arguably 2 minute intervals are tougher discipline for that. If you can look back on your day and see long single tasks without interruptions then you can be fairly sure you have been pretty productive.