I've been on Effexor XR for years now - mainly for depression. I haven't read anything about it helping fibromyalgia. However, research is finding that fm patients have low seratonin levels, so any medication that can increase them (including antidepressants) would probably help.
Just be really careful taking effexor. It has awful side effects when a dose is missed or late. When you stop taking it you could have alot if vertigo. For me I ad vertigo for the entire time i took it and it was much worse when i stopped. It took 4 months for my head to clear. I have a friend who had the same issue and she was very addicted to it.