I also used xpanel on one of my servers and had great success with it. It was able to handle close to 5,000 customers in the database before I had to delete some of the old accounts to make room on the server.
The only thing I didn't like about it was that there was no paypal recurring billing integrated into the script. Users had to login every month to pay which can be a turn off for many people. Other than that it works great.