If you have an iPod/iPhone (and there might be other podcasts out there that do the same thing) -
I used a Couch to 5K app, I don't remember what it cost but definitely less than $5. It does its thing in the background, marking the intervals, and you play your own music. I love it since I have one particular album that I have run to for *years* and for some reason I am still motivated by that particular music.
I would say the down-sides are:
1. Someone is telling you to run! Which I don't mind, I need some
2. If your music beat doesn't match up with the segment - e.g. really fast music when you're supposed to be walking or a pause between songs when you are running - it can be distracting.
But overall I really liked that it let me use my own music. One of the main reasons I bought an iPod last summer was so I could use this program.