You almost have to form your own conclusions. You'll get opinions from "they're crap" to "they're the best". Anyone who owns a few that work well will say they are great, anyone who has had problems will say otherwise.Just like with engines
I was about to buy a bunch on a cohort's recommendation, but since that I have seen them strip on a single aileron on a .15 size model, while they have also held up fine in other circumstances. I wonder about consistency same as I do with anything priced so low it seems too good to be true. If HS-55's are so bad, I wonder why my HS-55's are still working perfectly with no deadband on a 125mph electric which I regularly snap roll at high speed? Yet I recently lost one by bumping the rudder on the ground on a foamy. My conclusion is that HS-55's are decent servos, but the downside is that they can't handle much shock - they are prone to shock on the geartrain.
I am now a convert to HS-65's for peppy 1/2A size models and HS-85's for stuff with a bit more stress. I might yet try the HXT900's on stuff in which I don't have a lot of time or money invested. But anything scratch built with 15-20 hours in the airframe I'll pass.