I do ... with the OS running on it.  It's basically the best upgrade you can get right now, unless your system is over 4 years old or so.

As for build times, I have seen a boost of up to 4x, and sometimes more, but that's partly because our build process involves quite a bit of IO with file copying (a lot of unnecessary file copying, actually). 

For certain operations, like a tool I used to convert NT Build based sources/dirs./makefiles to vcxproj format, the time went from >24 hours to ~4 hours.