I was told about Under Armor when I started building my fett 2 years ago. I'd never tried wearing em, but thought if the fat guy in Resident Evil Umbrella Corp body armor in the middle of a South Carolina summer says it works, it must.
My input for you... There's a lot of these type clothes. I found the Starter brand at Walmart is less than half the price of Under Armor from a sporting goods store. Unfortunately, neither of em work for me. They just sort of pool the sweat around you and make you feel like you're wearing a soaking wet shirt while keeping the outer clothing layers dry.
What I use is the brand that's right next to the Under Armor stuff at sporting goods stores and it's the same price: Nike Pro Combat. Get em to fit tight and you'll swear you're not sweating at all. Can't believe I didn't find these before... I'm playing with a band and/or setting up DJ-karaoke equipment 3-5 times a week, and I won't do either without one of these shirts keeping me dry for the chore of loading equipment in/out of my car.