I once transitioned from LTE to EV-DO during a speed test. Obviously the speed dropped, but the speed test completed properly . There were no interruptions at all.
This was while leaving the LTE coverage area, so it would have been an orderly intentional handoff... in other words, not an unexpected loss of signal.
AT&T's marketing department says that their HSPA is faster than Verizon's EV-DO (which is true assuming that both networks are managed properly and one isn't overloaded). This means the transition is not as noticeable. It does not mean that there is anything bad about Verizon's network. There were issues early on, but these seem to be resolved now.
Of course, AT&T still has a lot of EDGE coverage in some areas, so the transition between HSPA and EDGE is very noticeable.