I haven't used Elos yet, but I've heard nothing but rave reviews about them -- the quality of reagents is high-quality lab grade, like Salifert, and they're supposed to be somewhat easier to discern than even the Salifert tests are... Price-wise, Salifert still has them beat, so I still choose them over Elos, but based on what I've heard and been told, I would trust an Elos kit just as highly.

-Corruption