I've been using the Ocean Master test kits by Instant Ocean for ph, nitrite, nitrates, ammonia and Alk. I use the API kit for the Calcium mostly, or to verify the results for some of the Ocean Master results, but they always match. My opinion, and (I'd like some feedback on this one), dont use the Red Sea Magnesium kit, its difficult, poorly documented and downright usesless. Matt at Austin Aqua farms uses the Elos Mag kit, says its great and easy.
What I dont like about the API kit is the vials. The lid for the vials goes on the outside, and when you shake it, then remove the cap, liquid always spills on the outside of the vial onto your fingers. I used a Hagen Calcium kit for the first time last night, and while I had 3 reagents to mix in, the vial had a lid that went on the inside of the vial, so you could shake it, and remove the lid slowly and the fluid would stay in the vial, not on the outside and on your fingers. My two cents.