They're generally very inaccurate. I don't bother testing anymore except for ammonia when cycling, so I don't even use any of my (too many) test kits.
If you wanted more accuracy, go with the API liquid tests. However, you have to decide what you want to use them for. If, for example, you only want to test for ammonia every once in awhile, and the strip will give you a decent idea if there's any measurable amount, then it's fine.