I came to Sprint from Mobi. Although you do get nationwide roaming, it's only for voice/text. No data roaming.
Service was pretty good. Coverage was a bit better than my current Sprint coverage. Hopefully that will change soon.
My problem with Mobi was the lack of high-end devices. Their current flagship phone is the S3 which was just released last week. In order to have a reasonably good piece of hardware, you'd have to buy a good used phone on Ebay or Craigslist, then pay $50-$75 dollars to have it flashed to Mobi. Okay for one person, but with a family of four, it just became a royal pain in the ass.
Last August, it was rumored that they were to be sold to Sumitomo, but no word on that ever came out afterwards. We left because I spending a small fortune on trading used phones, and they had absolutely no announced plans for LTE. I think they were able to build their infrastructure using Federal Rural grants, and now that those are dried up, they don't have the capital to build up their network.