Well heres my rundown for you. xAvia has been known for switching billing systems. Its really hard to find the perfect one, but with the money I've spent im settling with ModernBill:
Anyway heres my reviews:
Won't even go there
Support was OK. But upgrading always caused about 12 hours with no billing system, until John came online and fixed the internal server errors.
Support was 10/10 no question. Went away just not finding what we needed.
Had it in November. Quit it because it was in development. Came back again about 2 weeks ago, and I have to say its pretty good. Once they implement the feature of auto-creating reseller accounts, they will be golden IMO.