It’s a default font on Macs, but rare on PCs. Since it’s not technically web-safe, some people may have it and some people may not. If you want to use a font like that, without using @font-face, you may want to write it out several different ways because it might not work the same for everyone.
I like using a font stack that touches on all bases like this:
font-family: "HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue",
Helvetica, Arial, "Lucida Grande", sans-serif;
This recommended font-family stack is further described in this CSS-Tricks snippet Better Helvetica which uses a font-weight: 300; as well.