I realise it can be quite a speculative subject, but I believe JKR herself must have had some distinct image of Snape's teaching role in Hogwarts.
Evidences seem to be contradictive though.
We know Snape became a teacher to be close to Dumbledore, not because he wanted to teach. On the other hand, from the first year he asked the post of DADA teacher, meaning he actually was interested in teaching activity in general.
He is definitely a bullying, unjust and discouraging - not a good teacher's features exactly. But at the same time he pays a lot of attention to every student (even if negative), cares about all the details in class and seems to be interested in passing his knowledge of a subject to the fullest.
I would personally emphasise the Occlumency lessons. In my opinion Snape was surprisingly patient considering the circumstances and Harry's behaviour.
So has JKR ever said anything on Snape's competency as a teacher?