The best language teacher is a good teacher. I really hate schools who market their staff as 'native speakers' so they can overcharge. Natives can be the best, but they can also be the worst.
I have to agree with Vince here - I have had experience of 'native speakers' trying to teach a language, but because they don't understand how the language structure works, they have no idea how to teach it.
Well, this is hard to vote. Whilst a native speaker can speak good English doesn't mean he/she could know how to teach.