I have had a Virgin Mobile pay-as-you-go plan for about 7 years now.
They were great.......until the last 6 months. I've had 3 problems with them in that timeframe. Two were auto-topups not occurring when they should, one of them resulting in my cell phone service being turned off completely at an unfortunate time. The third was for outrageous charges for phone calls - instead of charging me the standard $.25/minute, they charged me $1.05/minute for a series of calls - thankfully I just happened to review my detailed call records one day and caught this!! They have never given me any explanation of why these problems happened, nor any assurance that the core problems have been resolved and won't happen again.
I find I need to monitor my account constantly these days, instead of it being on auto-pilot like it was for so many years. If there is as much as one more incident this year, I'm giving up and finding another carrier.
ETA: NEVER, NEVER report a problem to them via email. ALWAYS report it by phoning them. They do not take action on the emailed problems, even though they send emails saying they will. With my last problem, the sent me an email saying they were working on it, then apparently closed the case without fixing the problem or even notifying me....It was resolved only once I phoned and asked what the heck was going on with it.