The bottom line: The Icebreaker 150 Zone is a pricey but high-quality lightweight Merino baselayer suitable for year-round outdoor activities.
The Icebreaker 150 Zone is an 83% Merino wool long-sleeve baselayer intended for use in a wide range of temperatures. At 150g/m2, it's a fairly thin shirt, and likewise it is very light, at only 9.66 oz. Icebreaker call this shirt "ultralight" weight, and intend the shirt to be used even on warmer days. People tend to think that wool is for cold weather - but this shirt is actually pretty versatile. It breathes exceptionally well, so you won't sweat through it as easily. And when you do sweat through the shirt, it still retains much of its warmth. This means you can take it out even on summer hikes in the mountains.
The shirt comes in a half-zip and a crew neck. I have the half-zip. The zipper itself is pretty long, which makes it useful for temperature regulation during exercise. The collar comes up around the neck nicely. The sleeves have thumb holes, which are comfortable - although the sleeves are quite long (see below). The shirt's construction and materials feel solid and durable. Though the fabric is thin, it feels sturdy and I don't worry about rips or snags.
For the weight, the 150 Zone is quite warm. I bought the shirt mainly for hiking and running, and I will use the shirt for runs on days down to about 37F without a jacket. Running with the shirt above 45F will keep you pretty warm, but for less strenuous activities like hiking or walking, this shirt would probably be suitable for any temperature between 40 and 60 degrees F, depending on how hard you are working. If you want a warmer baselayer top for winter, consider one of Icebreaker's heavier shirts, such as either the 200 or 260 Zone.
The shirt has "heat dumping zones" under the armpits that are meant to be slightly more breathable and not as warm. This is meant to help keep you from sweating through the shirt as quickly. It's a nice feature, but I haven't noticed it too much, since this shirt is already pretty breathable. When running, the 150 Zone breathes great - the only location I usually get sweaty is in the elbows where the sweat can't evaporate.
The only thing left to mention about the 150 Zone is the fit, which is where it gets tricky. I am 6'0"ish and 165 pounds, and I wear a medium. It's tight around the arms and torso, which is what I want for a baselayer. However, the sleeves of the shirt are a bit long even on me. I expect that this would mean the shirt is either too tight on a more muscular build or too long in the arms on a shorter build. I'd recommend using the "Fit Finder" tool on the Icebreaker website, which takes into account your height, weight, how you want the shirt to fit, etc. to recommend your size, just to be safe.
If you can find one that fits you, I strongly recommend the Icebreaker 150 Zone. It's a comfortable, warm, but versatile shirt suitable for year-round use. At $100, it doesn't come cheap - but you're paying for the quality, and you can tell when you put it on.