Linux is It's free as in free speech.
There is no DRM, (Linux ignores DVD regions, and doesn't encrypt digital outputs)
There are no tailored updates that could introduce targeted backdoors.
Almost everything on the system updates in one click.
It's open source which leads to less bugs and more security. Those flaws that are found are quickly patched and distributed.
You don't have to mess with the drivers if you don't want to. If it can work, it usually just works.
No artificial limitations or anti-features. It can read more than one partition on a USB drive, and make a RAID array out of whatever you want to.
It supports all sorts of complex routing and network protocols.
Performance doesn't lag as a system ages.
Variety, there are over 200 distributions of Linux and counting. Many are different options for a general purpose computer, and many are specialized for one purpose (say a router, firewall or music server)
Modularity and small base footprint means you can have a modern OS on ancient hardware.
Overall I like linux because it treats you like a person, and not some wad of cash to be harvested.
There are of course some cons, :
1. It's not in the mainstream, meaning you will be left out of some things unless you dual boot or have another computer.
2. There's a learning curve. Certainly doable now for an average power user. If you're the one who figured out how to get rid of the last infection your computer had, you can probably figure it out if you're willing to read the documentation. (If you've never used a computer this really isn't a factor.)
To respond to specific criticisms... The Morris Worm was 6 years before the BSD OS was open sources, 3 years before Linux Torvalds wote some basic code to explore the functionality of the 8086 processor, and 6 years before Linux 1.0, the first non-beta version.
Open source actually makes programs more secure. Linux has been approved for governmental use at all levels, and was the first kernel where things like stack and heap randomization could be implemented. It was also the first to support mandatory access control (. Because it is open source so many people can work on it and it will stay the cutting edge for new security models. Last summer a fuzzer discovered 6 bugs in the web kit engine, and around 200 in the IE engine.
There are distribution of linux that will run in under 64 MB of RAM, or entirely within 128 MB of RAM with no harddisk space needed. And unlike XP they include the newest kernels, networking programs and tools, making it much more secure.
As for being a waste of time, it's best to buy hardware with linux in mind rather than having it as an afterthought. I'm sure several manufacturers would release dual boot systems that had drivers for all of the hardware in the linux kernel tree, if it weren't for the windows OEM agreements that prohibits such practices.
MATLAB has a native linux version, that can also run of the BSD's.
The Disney corporation pays the developers of WINE to make sure the latest photo-shop will work.
I don't think AutoCad does, but that brings up the next point. It may very well be a waste of time for some people, but not for everybody. Millions of people like myself have make the transition and have been very happy for it.
Some times for successfull migration
http://www.psychocats.net/ubuntucat/a-home-users-successful-migration-strategy-from-windows-to-ubuntu/