I can't speak for Icebreaker, but having used Smartwool base layers, I can speak for merino wool base layers in general.
My micro weight(150g/m^2) long sleeve is one of my favorite pieces of clothing. For here in California I wear it as my first layer, even if the sun is beating on me at 10,000ft on an exposed pass. It rarely gets so hot that I switch to a short sleeve button up synthetic shirt. If it's a little too cool for the baselayer alone, I add the afore mentioned short sleeve synthetic, on top of the smartwool. As it gets colder I add a hooded down sweater/jacket, then my rain jacket if it's really cold.
For 3 season backpacking in CA, that's all I need. If the temperature range was going to be cooler, I imagine moving to a light weight(195g/m^2) or medium weight(250) would be all that I would need to change to stay pretty comfortable.
For non backpacking outdoor activities I often find my smartwool and a wool flannel keeps me pretty comfortable.
The fact that it doesn't stink after I've been sweating in it all day, really is a selling point, but the fact that I can be comfortable in it when it's warm/hot out, and that it keeps me warm as a baselayer when it's cool/cold out, just makes it a hugely versatile piece of gear. Possibly my single favorite article of clothing...
I think I should also not that it is a distinctively different piece of clothing than sweaters or flannels you might pick up at the thrift store.
I love my sweaters and flannels from the thrift, but a light weight wool baselayer can do things that they cannot. Most certainly worth the price tag.
My philosophy:
Baselayer: Pay up, it's worth every penny, because you won't find it at the thrift.
Main layer: Thrift store, too easy to find what you need, no sense in paying full price.
Jacket/outer: Pay up, I rarely find useable outerwear at thrifts, and you certainly won't find 5oz. down jackets, or quality wool pullovers...
Rain gear: Go cheap, go frogg toggs.