T-Mobile's 4G is real 4G, as is LTE and WiMax. Prior to last month, none of the so-called 4G networks could be considered real 4G; however, the ITU recently changed the definition of the 4G standard such that HSPA+, LTE, and WiMax are all 4G networks.
In all honesty, T-Mobile has the largest and fastest 4G network, even faster than Verizon's tiny LTE network; nonetheless, I still feel that T-Mobile, despite having the most 4G coverage and fastest speeds, has the worst 4G network. This is because HSPA+ does very little to improve network latencies, to which both LTE and WiMax provide significant improvements.
I feel that 5mbps is wholly adequate for nearly any mobile application, so the speed increases seen in 4G networks are mostly unnecessary. Speeds above 5mbps will have essentially no affect on the mobile data experience. In order for further progress to be made on the mobile data front at this point, latencies need to be reduced near broadband equivalents.