Look at "About LimeWire"--it should say Pro there. You'll not see much difference between the free and the pro, but you'll know you're helping the project. You are also entitled to free email support as a Pro user, but I've found the help here to be far better.

Cheers.
btw--check your firewall (and router if you use one) to allow Gnutella/LimeWire on port 6346 if you want to share and get better results.