I did it in the spring. With rates at ~3% and P/E at 18 or whatever it was, it was a no brainer. Took as much money as they'd give me (which wasn't much, to be fair - mortgage went from $250k to $360k) and dumped the excess into the market.
I wouldn't do it now, probably, just because stock prices are so insane, but you could make an argument for doing it anyway if your timeframe is 30 years, or alternately if you're worried about inflation.
-W