I've done some high intensity interval training (or crossfit, whatever), and it's good stuff. I do better when I have someone telling me what to do and pushing me, so it worked out well. Just like p90x, most people quit after the first week. They are great and rewarding workouts, but go into it knowing that for the first 1-2 weeks, you're going to feel like you were run over by a truck. If you can push through it and pay attention to your diet a bit, your body will adjust and after a few weeks you'll really start to see results.
In my first month (3 times per week) it took 2 minutes off of my 5k time, which is something since we never really did any longer runs (1.5-2 miles total over 90 minutes--but with 20-40# packs on).
If you do it, be prepared to be a mess for a few days, and stick with it.