I bought the Sera master kit, and was pretty happy with it. The only thing I had trouble with was the KH and GH titration (but I've always had issues with titration...), and I suspect that is because my water tends to be pretty soft to begin with.
I recently exhausted one of the reagents on both the NO3 and NH4 kits, which is kinda frustrating, as the other reagents seem mostly full. I did get two complete tank cyclings, plus a handful of other results, so I can't really complain too much.
Recently bought the API kit until I get something else ordered, and it does seem a bit less accurate and harder to determine the colors then the Sera kit.
The Sera NO3 kit might be more work then the API (not certain about that with all the shaking involved with the API one), and I'm dubious of how precise my measuring of the dry reagent was with the little spoon, but the results seemed to be fairly accurate and consistent during cycling.
I've got a Nutrafin kit on order, so I'll get to see how that compares, and maybe I'll try some Elos kits next...