It sounds like a good technique if you have trouble focusing and feel unmotivated. However it feels like a lousy technique if you are :) When you're in flow-mode you don't want to have some stupid clock disturb your concentration every 25 minutes. You want to look down at your watch and go "whoops, I've spent 6 hours coding"
I think people overestimate the value of flow. You need to step out of that state regularly to put on your big-picture hat and access the work you have been doing. Pomodoro is not just about being productive, but being productive at the right thing.
I've noticed that even in the flow I welcome the disturbance in concentration. It gives me a chance to step back and take a few moments to analyze whether what I'm doing is actually the best way of doing it.
It also often happens, unfortunately, that taking a 6 hour swath of time to do a single thing is simply a luxury I can't afford.
