Capilene is Patagonia's trademark name for a polypropylene fiber made by Carmel Olefins. It is a moisture-wicking fiber in which the inner core "filament yarns" absorb moisture -- wicking it away from the body while the outer layer distributes moisture, allowing it to evaporate away.
Fabrics using capilene tend to also include spandex to increase the ease of movement for the wearer. One of the primary values of capilene garments is their inter-layerability. Capilene "long johns" can be worn alone when cool, under regular clothing when cold, and with other high-tech fabrics under more extreme conditions.
Two of the neater uses that capilene garments experience are use by astronauts and drysuit SCUBA divers.
Popular reviews found at the time of this writing are predominantly strong praise. My personal experience with the fabric concurs. I enjoy the warmth, the moisture management, and the flexibility. Capilene is machine wash- and dry-able and cleans up nicely.
The two detrimental features that I've identified are the expense and the flammability. So don't get too close to the fire!