The biggest pros are that it's free, safe and secure, and infinitely customizable.
Because it's modular in design, as opposed to window's monolithic architecture, it's very hard to infect a linux machine, and hard to do any damage even if one is infected. The modular design also makes it less likely to crash, whereas a single problem in a single windows program can result in a blue screen of death.
Because all the code is open source, you can see what's there, and change it however you want, so long as you don't try to make money off of it. It's also less "bloated" than windows, allowing superior performance, even on older machines.
The downsides are that most people have used windows their entire life, and learning a new system is sometimes frustrating.
Some windows programs will not run on linux (though a surprising number will), but most have a free linux compatible alternative, such as OpenOffice instead of Works, GIMP instead of Photoshop, MPlayer instead of whatever windows uses these days, etc.
For most hardware, linux has superior plug and play, but if you have a problem with a piece of hardware, support for windows is usually easier to find than it is for linux.
I made the switch about 4 years ago. After the steep 3 week learning curve, I never regretted it. I've also never had a single virus, rootkit, or malware installation since switching, and can leave my computer on for months without it crashing.