I get about 3.5 - 4.5 MBs in Minneapolis with 1 -2 bars of T-Mobile service. I stuck my AT&T SIM in the Nexus and only pulled 1.5 - 2.5 on 4 bars. I never really get 4g speeds on AT&T on the HSPA network, regardless of device, but my Galaxy Note typically does about 3.5 MBs in the same location, so the Nexus doesn't seem quite as fast on AT&T. IMO 3.5 - 4.5 is plenty fast for most everything I need to do mobile with my device, so I'm not complaining, but my VZW Nexus is regularly about 14.5 MBs down, and frequently up around 18 in the same spot, with only 2 bars!
Never had a T-Mobile device that offered 42 MBs HSPA, so can't compare the Nexus to anything like that though.