I thought people had given up on that moderately preposterous belief years ago, but I guess it's still around. The answer, of course, is that it's a perfectly serious computer. I'd say anything with a full UNIX environment on hand and full multiuser UNIX server capabilities installed out of the box is a lot more "serious" than what Windows will give you in the consumer version, if not the Server version as well.
Not universally perfect--not every piece of special-purpose or industrial software is available for the MacOS, and Apple doesn't make industrial or embedded computers required for certain things--but then, Windows has support gaps as well, depending on what you want to do.
Huh? Gaming, you'd be hard to argue Windows doesn't have an advantage, but last I checked the average office worker rarely required much more than a word processor, spreadsheet, Powerpoint-equivalent, and maybe a database and calendaring app. All of which are available in a variety of flavors on the Mac. Heck, MS Office--which used to be the definition of what an "office worker" needed--is better on the Mac than Windows, if you ask me. There are less CRM packages available for the Mac, but those are used by a tiny minority of office workers, and most of the recent solutions are web-based anyway.
You want "serious," I work at an energy research lab where we have twenty people and some bonus grad students doing heavy scientific number crunching, data analysis, data visualization, mathematical modeling, technical presentations, publishing papers, applying for patents, and more. Macs at desktops outnumber PCs 5 to 1, and although I use Windows-based machines for the data collection and process control software I write, so I can buy industrial motherboards or rackmount systems with a half-dozen available motherboard slots, the software package IS available for the Mac. In fact, due to the push toward USB-based DAQ hardware, it's actually getting EASIER to do things Mac-based than in the recent past (farther back, we had 68k Macs for the same purpose), and I may well consider switching some of our systems over eventually. Oh, our server is also a Mac.