As far as I know, Seroquel is supposed to have an off-label antidepressant effect for bipolar depression. I take only 25mg for sleep, but I used to be on a higher dose, and was for a number of years. It's difficult for me to discern whether Seroquel gave me an AD effect, or whether it would have been another med (definitely a downside of a medication "cocktail"- I often don't know which med is doing what).
I will say that I've been taking the nightly 25mg every night for a short time and have felt noticeably less depressed. I thought it was a decrease in Lamictal, but maybe it's the Seroquel!
Bottom line, if it's helping your depression, that is fantastic!