After several years of working on Windows XP machines, I've recently made the switch back to Mac.
The difference is profound and makes me regret that I didn't push for the Mac a long time ago.
I build things that run on web sites and those sites all run on Linux. When I have to work on a Windows machine, the only way I could really do development was to use a Virtual Machine which let me effectively have a Linux server running "inside" Windows.
While this works, it means constantly jumping through a lot of hoops. It also means everything runs slower because hardware that was designed to be used as a single computer is being asked to do the work of two or more.
With a Mac these issues simply go away.
Under the hood, a Mac works basically the same as the servers I write code for. This allows me to do what is called "Native Development". No more jumping through hoops. No more forcing hardware to pull multiple-duty.
Take this along with the improvements Apple made to the Mac's operating system since 2001 (which is when Windows XP was released) and I'd guess I'm 25-30% more efficient on a Mac.
It's good to be back.