Just throwing in my 2 cents..

The Hagen/Nutrafin test are dreadful. The colour chart is terribly
unhelpful and fades over time, so the bad match you already had ust
gets worse. Their #3 reagent bottles were also defective for a long
time.
I don't really like Tetra tests because of their droppers (then again
they may have changed them in the past few years) - the drops come too
easily and too fast.
I picked up an Aquarium Pharmaceuticals kit and so far I love it. The
colour chart is easy to read (gets a little more tricky above
30-40ppm, but by then you probably want to be getting the siphon
anyways). The charts are LAMINATED, so they won't discolour, and thus
far they've seemed very accurate in both fresh and sal****er. I never
know what chart to use for my brackish tank though. :P