I tend to use neem on plants prone to spider mite infestation. But it's more of a preventative measure.
When the mites present themselves, I try to spray off as many as possible with a directed stream of water from the hose, and then apply a more concentrated Azadirachtin-based pesticide like Azamax and Azatrol. I find them more effective (but also more expensive) than simple neem oil.
Last year I had spider mites show up on some tomatoes right before harvest, and because I was reluctant to spray anything - even neem oil - I released predatory mites with surprising success.
Here's a link for the ones I used:
Phytoline p
Good luck!