I'm a fairly tall guy, but very long in the torso, so most shirts have a tendency to untuck themselves.  I can find dress shirts long enough, but the typical undershirt has a tendency to slip out and then bunch at the beltline.

These shirts are long, long enough to stay tucked in all day.  The cotton is on the thin side, which is fine because I'm not looking for insulation.  The biggest thing for me is the neckband - too often manufacturers will use a bad seam on the neckband, creating a spot that rubs, or they use those awful tags that itch.  These shirts have neither - the neckband seems to be a single piece (or has a very nice stitching job) and there is no tag.  No itching, no chafing.  So, affordable and comfortable T-shirts?  Yeah, that's 5 stars.