Since everyone is telling their "conversion" story, I'll throw mine in.
I'm a graduate student currently working on a PhD in Analytical Chemistry. The chemistry community is actually pretty mixed. Most analytical chemists and inorganic chemists tend toward Windows, while Organic and Biochemistry have a heavy Mac presence(although far from universal). Physical chemist are the odd ones out-physical chemistry and computational chemistry are almost one in the same, which implies a heavy reliance on *nix systems. Linux is popular, although OS X has made a lot of inroads. Whatever the case, Windows is pretty universal for most instrument software now(except for the rare one that is Mac classic or *nix specific-our NMRs are a mix of these), so you will find yourself interacting with Windows regardless of what you use for your personal platform.
In any case, for day to day work I was a pretty heavy Windows user until March 21, 2012. My only real Mac experience up to that point had been with iMac G3s in high school(hated those things at the time), a B&W G3 for Adobe Pagemaker in journalism class, and a couple of G4 towers in the computer lab at my undergrad(not sure what models, but they were graphite cased) that I used very rarely because they could print for free

.
On March 21, 2012, however, I was working on my literature seminar on my year-old Toshiba laptop running Windows 7. The literature seminar is one of the "milestones" in our program that one must present, orally defend, and pass in order to continue on in the program-in other words it's a big deal. I was working on the Powerpoint one evening at about 10:00, and my computer shut off. Apparently, somehow or another, my Windows install had gotten corrupted, and I couldn't get the computer to reboot.
The next day, a friend lent me his "old" Mac-an Aluminum Unibody Macbook running Snow Leopard. I pulled the hard drive out of my Windows laptop, and another friend stuck it in his desktop and pulled my Lit Seminar files off of it(the powerpoint and papers I had downloaded). I spent the next few weeks learning my way around OS X as a "trial by fire," but presented and passed my literature seminar one week later on March 29. Two days after, I went to the Apple store in Lexington, KY and bought my first Mac-a 13" Macbook Pro. As it so happened, on that particular day, the University of Kentucky(which I support) and the University of Louisville(where I attend) were playing each other in the Final Four of the NCAA Tournament. I was a bit self-conscious showing my U of L student ID in Lexington to get the student discount

(UK won, by the way).
Fast forward to the spring of 2014, and my faithful Macbook Pro started giving me problems. It was very slow, and generally just unuseable. I took it to the Apple store in Lexington again, where it took them a week to figure out that the hard drive was bad

. In that week, I had to dig out the same Toshiba laptop that had failed me before(since repaired) and use it.
That was the excuse I needed to buy another Mac. I went on Craigslist and bought my first PowerPC Mac, a Powermac G5 single 1.8. The rest, as they say, is history. I'm a collector by nature, and unlike some of the other things I collect, G3/G4/G5 era PPC Macs have the unfortunate combination of being relatively available and fairly inexpensive(for the most part). I'm up to about 40 PPC Macs now, and have developed a particular interest in G4 Towers. I've also stretched back into 601-604 based computers(primarily after being given some), as well as 68K Macs, although admittedly my interest in those isn't as strong as the early 2000s era computers.
I also have to thank the tremendous amount of knowledge available here-folks like Intell, Altemose, Gavinstubbs, 128Keaton, Eyeyougren, and I'm sure others that I'm(unintentionally) forgetting. They have advised me and led me in the right direction. I've taken their advice and used it to "get my hands dirty" and actually see for myself what works and what doesn't. I can't think the folks here enough for that. I've tried to-in turn-share what I've learned in my experience with these older-but still very capable-systems.