I've been using United Mobile for a couple of years with great results. I've only had to call customer service a couple times in that span but both times the service was great. There have been times when the call quality was less than perfect, but by and large no worse than many other providers I've used. If you use them with a service such as CallBackWorld.com you can save quite a bit of money all calls back to the US.

I've also used Telestial to purchase a local SIM I couldn't find elsewhere (needed the number before I left) and I had no complaints. Yes, there are cheaper alternatives in many cases, but for what they do I don't think their prices are out of line. If you are comfortable buying from a vendor on eBay (and can find what you need), then go for it. If you can wait until you are "in country" then buy the SIM when you arrive. But if you don't fit those categories then I wouldn't have problems recommending Telestial.

I will say, I have never bought "top-ups" from them -- I always buy those from local vendors when I need them.