I did the same plunge 3 years ago. At that time, I had 8 years experience in IT.
One thing I did, in addition to having a good contract BEFORE I left, was to take a one year without pay. This way, if something didn't work out the way I was thinking, I could go back to my previous job.
BTW, the hardest thing for my as a contractor now is training. I used to have 1 week a year of paid training/conference. Now, I obviously pay everything myself. For example, a SANS course at roughly $4000 + travel $1200 + lost in revenue of $4000 = $9200 for a week long course!!! It used to be free...
You also need an accountant ($2000/yr), health plan in the US (I leave in Canada!), dental plan, put money aside for a pension, etc.
BUT, I love changing place often, meeting new people all the time, touching different technologies, etc.
Bottom line, I wouldn't go back as an employee who more often than not, stay in the same office for years, doing more the less the same things every day... (ok, I am pushing a bit, but...)
Hope that helped!
OSCP, GPEN, GWAPT, GSEC, CEH, CISSP