I've used them in the past, wasn't a huge fan, and I feel like their updates were a little slow. Haven't used them for over 1 year 1/2. Not sure if newer version came out.
I think right now nothing comes close to WHMCS, paid or not, the new clientexec looks a lot better than it was and license fee's are much cheaper, but in the free realm its really lagging behind the times now. If your hosting/providing services for money, I think a quality, paid billing solution should be very very high up your priorities list.
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