Any old test kit will yield any old result. The crap kits at your local Petsmart are truly that... crap.
Generally speaking, the best ones are the Salifert and the LaMotte. Middle of the road are the Seachem and FastTEST, bottom of the line are the Tetra, Aquarium Pharm., etc.
It's not that the cheap kits cannot provide an accurate result, it's just that it's very hard to do so. A good kit makes it very easy to portion the right amount of water and makes the colors, times, etc very easy to interpret.
Personally, I use the Seachem kits. They are a great balance of price and performance. My LFS uses Tetra, and literally told me to stop bringing in my water because I have better kits than they do, so I can do better testing myself. I've used FastTEST as well, but I find their color charts really difficult to interpret.
You can get the Seachem Marine Basic online. If you have a reef, I'd keep a Reef Special and Reef Calcium on hand as well.
Oh yeah... and the cheap hydrometers are fine to start with, especially if you buy your water pre-mixed, but if you really get into the hobby, put a refractometer on your short list of things to get in the near future.