I meant to post about this when I got back from my trip to Idaho, but life got busy. Before we headed to Boise, I picked up the packable down jacket and two Heattech tops at Uniqlo - one camisole and one long-sleeved scoop-neck t-shirt. The jacket performed admirably - it really does pack down in a small stuff bag, and it was nice to have a second coat option without having to pack and haul around two full-sized coats. It isn't super-warm - it wouldn't do for proper winter when the wind's really blowing, unless you layered it under something else. But in 30s-50s temps, going in and out from the car and buildings, it was a nice weight and not a pain to carry around.
I was less impressed with the Heattech. I wore both pieces under tops and sweaters, and honestly they didn't seem any warmer than the plain knit camisoles I have from Target. I definitely wouldn't wear them for any occasion where I'd be outside for long periods. Uniqlo does have a 'super-warm' version of heattech that I haven't tried.
I had a hard time finding the XLs, even though they do nominally carry them in stores. If I wanted an XXL - which would be nice if I were going to wear the jacket over several layers of sweater/top - I'd have to order online. I guess Uniqlo's prices are good - I paid ~$50 for the jacket and less than $20 for each top - but I'd be more likely to spend a bit more and get something more substantial from Lands' End, Winter Silks, North Face, etc.