We have used WHMCS but we decided to try Blesta and until now we are very happy.
However, it all depend your needs. We have the basic management and ticket needs and it works very well. The only down is the domain manager.
I know the blesta team is working on this and we should see some improvement in future versions.
BTW, cPanel extended module is a must and our clients love the fact they can use all their services directly from the client area.