I was introduced to Icebreaker during a NOLS semester in New Zealand in 2008. I wore my first Tech T for those 80 days straight, and I've been an addict since.
After the first wash, these are the softest t-shirts I've ever worn. I wear them alone to stay cool during the summer, and I'm wearing one right now as a base layer under a heavier Icebreaker zip. I wear them around the house, to the office, on my runs, on top of mountains, while traveling, and just about any time I'm wearing a shirt at all. The only cotton t-shirts I still own are undershirts to wear with dress clothes. Heck, if Icebreaker ever make a white Tech T, I'll dump those few remaining undershirts, too!
One weakness is their relative delicacy in the wash. These things have held up for me through hell and high water in the backcountry, but you really do need to heed the care instructions when it comes to washing and drying. Repeated trips through the washer and dryer will make for a sad Tech T, often with holes in the upper back where the fabric is stressed. As long as I air dry my shirts, though, they remain perfectly happy. It's not a deal-breaker for me, as putting in some extra effort to take good care of my quality gear is rewarding.
They really do resist odor as advertised. Realistically, I can wash one at the beginning of the week, run in it a few times, and it'll still smell presentable and feel soft at the end of the week. They're the polar opposite of Patagonia's Capilene shirts, which I also run in--they're more or less unbearable after a single run.
I like a snug fit, so I order a size down. I'm 5'6" and about 150 pounds, and a small is very comfortable.
Buy one of these now, gradually grab new colors as they're released, and take care of them! They've done me good.