I like the lowers on my ultra classic. I bought the quick attach kit for them. This year I took them off for the summer and put them back on a few weeks ago. I personally feel cooler with them off, but according to my temp gauge the bike runs a little cooler with them on. By a little I mean that the I can just tell that my oil temp gauge is in a slightly different spot. So, I would not say it is significant and having me feel cooler takes priority.
The sound of the engine changes with and without them, so don't think that something is immediately wrong if you hear different things.
They don't do a great job off keeping your feet dry in rain since you water hits in every direction, but they do stop the direct attact of water on your lower legs, similar to a windshield.
With them on they make checking some of the critical fasteners and changing the oil filter a little more difficult, but not too much. About the only thing I store in them is a puck for my kickstand on soft ground and a couple of microfiber towels.