I sort of switched earlier this year. I replaced my Windows laptop (the only computer I have) with a Macbook.
1- How difficult was it to learn how to use a Mac after using a PC for years?
The general stuff? Not bad. There were/are little things, though, that did and DO drive me up a wall. Some of the keyboard commands are not intuitive or easy to remember for me at all. I had to google how to do a print screen, and even after following the directions it took forever for me to realize that it saved the images to the desktop instead of saving it in my clipboard.
I also flat out detest how Finder only lets you drag and drop or copy and paste, but not cut and paste. I’ve gotten around it by dragging the files to the desktop, going to the folder I want to move them to, and dragging them there, but I still think it’s monumentally stupid to not be able to cut, dammit!
2- How difficult is it to transfer your files from PC to Mac? (I have lots of MS Office documents [mainly Word and PPT] and a gazillion pictures I want to move)
Haven’t really gotten around to doing this (I have an external harddrive with my files saved, but I haven’t really needed to use any of them yet).
3- Generally speaking, are you glad you switched and why or why not?
Meh, ambivalent. I’m glad that I figured I’d learn a new OS, but I don’t think it’s the bee’s knees. Some things it does better (IMO of course) than Windows and other things I can’t stand. Granted, I haven’t really tried to be a power user or anything, so I’m sure it’s possible there are solutions to my irritations, but if they’re supposed to be so “intuitive”, why haven’t I figured them out already? 
I don’t like the almost condescending impression I get from it, though. “Software Updates!” bounces on my dock. I click on it and it tells me there are updates (duh), and more info just tells me what programs need to be updated. What, specifically, is it going to do for those individual programs, dammit? I get a feeling of, “there there, don’t you worry your pretty lil’ head about these complex software updates! Just let this Mac take care of it alllll for youuuuu”.
I’ll be getting a desktop in a few months, after I get married and we’re in a 2 bedroom flat instead of the 380 sq foot studio I’m currently in, and I already know it’ll be Windows (though I’m going with XP since I don’t personally care for Vista. It’s much much improved from when it debuted, but it’s not my cuppa). It doesn’t mean I’ll be getting rid of the Macbook; just that I really don’t see Mac as superior.