Loosing the monthly corporate discount is not that big of a deal for my wife and I since it wasn't that large (8%) compared to what some people were getting, but it is irritating, and so it made me revisit our options over the weekend. Since we like to buy our phones up front, and neither of us use much data, T-Mobile still works out cheaper than AT&T (or Verizon), even with the discounts offered for my company, but I'm surprised at how much more competitive their current plans are. Actually, at the 10Gb shared data level, AT&T with my corporate discount would be less expensive for us than two 5GB T-Mobile lines. We don't need that much data, but I also found an AT&T MVNO that may be a really good fit for us, and save us maybe $20/month off the base T-Mobile plans. I need to do a bit more research on that.
I think T-Mobile is going to regret not at least grandfathering in those already getting the discounts; it's one thing to stop offering benefits for new customers, but taking something away from existing customers is just going to drive them to look elsewhere.