Having jumped on the Underarmour train 6 years ago, and having tried just about all the clones since trying to find a cheaper alternative, including Nike, Adidas, Russell, the Wal-mart knockoffs, etc., I can testify that the Underarmour goods are the best.
I have two teenage sons, one now playing college football after 6 years wearing Underarmour for football, and another who has worn it through 5 seasons. In addition, my wife and daugthers wear the ColdGear and HotGear versions to work out.
The clones work, but they tear more easily, and do not stand up to the abuse the originals take.
As far as the ColdGear versions go, I like them better than traditional LongJohns, or even silk versions. They seem to retain heat better, while allowing moisture to evaporate more easily, keeping you from the wet and cold situation that happens with vigorous outdoor activity.
I currently have Coldgear and AllSeason gear that is 4-5 years old and still in great shape.
The ONE drawback to Underarmour style gear, and to all of the clones, is that that microfiber can snag and run easily, so going up against rough, abrasive surfaces can snag the material. It doesn't seem to compromise the use, just the looks, though. If you wear it under a cotton/wool shirt, that problem is no longer a problem.
Shop for the best price, especially at stores like Big5 and ****s, where sales, though infrequent, can save you some money.
Bill