We use Verizon for all of our employees. Which means that I've had to hand out a lot of mifis to those that have complained about this issue and provide a reasonable business justification for need voice/data (pretty much everyone in sales). But it also means Verizons already high rates are even more expensive with all the mifis we have as well.
We just recently tried out AT&T with some of our non-traveling employees. It's nothing short of a disaster. Complaints about lesser quality of service, coverage, dropped calls, and such. AT&T's business support is shockingly incompetent (something Verizon really shined at). And our AT&T sales rep is the Prince of Darkness, evil and nothing but lies come out of his mouth.
Already had to convert some of the people we just switched to AT&T back to Verizon. We won't be going any further with AT&T as a business.