I have been playing 7-string guitar for about 2 1/2 years. I have a RG2027, which I've owned for almost 2. I love it, and find playing a 6-string guitar now to be a rather odd experience! The neck is so... skinny!
I have been playing guitar for 12 years, and I got into drop tunings a LONG time ago. It always seemed like I was wanting a lower range, but, like Darren, I didn't want to get rid of my treble register. I settled into my standard tuning about 10 years ago, everything down a half step, then the low E dropped to D (which makes it technically D flat.) Cool, but I wanted more. Also, adding to the fact, I was influenced by studying classical guitar in college. Two factors
A) Classical guitar necks are so much wider than most electric 6-strings. I immediately found this more comfortable. The 7-string neck is closer to the classical width.
B) I had a seminar/collegium with a great classical player, named Paul Galbraith. He is a very accomplished Bach stylist, and he plays on a custom built 8-string (which he plays like a cello!) I was fascinated.
All these things put me on the road to 7-stringdom. Now that I answered that question with a dang book!