So far, ive found reaction fire quite useful, most notably on a very open desert map where three got plasma-blasts from reaction fire, plus one i only spotted when i turned a soldier to face towards the map boundary and found one had darted down the one-tile corridor between my x-naut's vision and the boundary and was three tiles or so from wreaking havoc.
Currently i find snipers more challenging in combat. Now i know how ET feels when he is killed from across the map when only a grunt with a shield can see him