If you don’t have a fast 50mm lens in your bag, you need one. I’ve got an earlier model of this lens and it never leaves my camera bag. It’s such a small and light lens, and the fast f/1.8 speed makes it the go to lens for really low light.

The other side is I’m a zoom lens guy so it doesn’t get as much use as my Sigma 24-70, but when this little guy is needed it’s needed.

There is a more expensive f/1.4 version of this lens. Never tried it personally, but I’ve also never had a need for a faster lens than f/1.8.  Unless it’s just crazy dark I’ve never shot wide open at f/1.8 anyway except to try it out when I first got the lens. The speed at which it focuses is very nice though.