I just picked up a 40 for the holidays, in between getting all new lenses and had nothing. I shoot mostly outside and on the move, so the 40's size and build carries major upsides ... the price doesn't hurt either.
It's a great little lens, sharp all the way to 2.8. For $139 @ B&H, how can you not? That's knocking on the door of the Nifty Fifty/Lego Lens! I'm more a results shooter, and it more than carries it's weight there. But the cost, size, build, make it something I think I'll always have in my pocket.
Regarding the speed in low light, I wouldn't make it my choice lens, the alternatives are going to be a serious price adjustment. Is it worth $200 to get from 2.8 to 2.0? Is the 35 at the top of it's game at 2.0? Never had one, but I'd seriously look at those questions.