Some people say the test strips are useless, but I believe they're still better than nothing at all. However, last time I was in that section of a Petsmart, the API ammonia test strips ran $15 for a box of 25 strips.
For the same $15, you can buy API's Liquid Freshwater Master test kit at an online aquarium supply site. The kit will test your pH, high level pH (if you ever get into keeping African cichlids this is very useful), ammonia, nitrite and nitrate. The liquid tests are much more accurate than the strips, not to mention you get many more tests out of a liquid kit than those strips.
I suggest using the online supply company because that test kit can go for double that price in stores.