I have BP II and used to take Effexor paired with Depakote. I was originally prescribed the effexor for depression and anxiety and was later diagnosed with BP. In all honesty - the effexor worked for me when i needed it, but I wouldn't recommend it for people with BP as a first choice. I was in such a state when I first started it that anything would have been a positive change but knowing what I know now about my diagnosis I would have tried other options. For me, finding the proper dose of depakote was the most important since it doesn't have a lot of side effects and keeps me on an even keel so I'm less reliant on other medication.
I discontinued the effexor, with GREAT difficulty after my pdoc and I had agreed that I had reached a point where I didn't need it and it was worth the effort to try to go without or use another med. I'm currently on a single dose of depakote and its working well.
The effexor worked really well for me with the anxiety and the depression, and is generally well tolerated, but it easily sends you into a manic mode if you're not careful, or don't react well to it.
Too, you might get some more info from your doc on her comment regarding the seasonal connection to Bipolar. Most people with Bipolar are ultrasensitive to the seasons but you may find that other treatments for SAD, (i think that I read you are using a full spectrum light), might help make any meds you are currently taking, or might switch to in the future, work better.
Good luck and take care,