I've been with Sprint since 4S launch.
While they have indeed been struggling horribly with data speed issues, the light is at the end of the tunnel, for me at least:
Not bad for an experimental LTE signal not yet officially announced as a active market yet.
I'm not by any means a Sprint fanboy, but over the course of the past 18 months, with all I've learned and experienced, I really feel that they are the best option for me.
I also think that while VZN and AT&T are currently at the top of the LTE signal market now, that will change dramatically once nationwide Network Vision is complete. Sprint has rebuilt the infrastructure from the ground up, not just bought the correct frequencies and threw up appropriate panels.
Once VZN and AT&T have saturation, they will be in the same position Sprint was in pre NV, an overburdened network with little hope for relief other than a complete network build out.
I realize that there are many factors, and I've oversimplified some of the above, but for me, I feel comfortable with my decision to stick with Sprint and have a lot of hope for a long, inexpensive and smoking fast LTE future with them.