It's easier to work with a surrogacy agency than to go independent, but I've done both. Independent was pretty good, but took a longer time to make my matches and get all the paperwork sorted.
If you do go independent, you're best off knowing a good lawyer to deal with all the paperwork BEFORE you start to make a match. The IPs feel better knowing that you've done the homework on this and you're not scrambling to find just any old lawyer. I recommend looking for family law practices since they will at least know of someone who deals with surrogacy.
I've had really positive experiences for the most part.
Chelle