I have Red Sea calcium and magnesium test kits. The only thing I don't like is that their standard calcium test only measures in increments of 50 ppm. They make a more sensitive kit, which I wasn't aware of when I bought the one I have.
I have Aqurium Pharmacuticals (API) test kits for NO3, PO4 (pretty useless -- not sensitive enough for reef use), KH, and Ca along with TetraTest kits for KH and pH. The API Ca test reads close to the Red Sea kit (although hard to say how close since one reads in increments of 20 ppm and the other in 50ppm), but for some reason the API and TetraTest KH kits don't agree very well. Today, for example, the API KH kit said 11 dKH while the TetraTest KH kit said 9.0 dKH.
Salifert kits are suppose to be the best, and I'd hope so considering how much they cost, but I've yet to try them.