I had an ultrasound on my kidneys and it was just like any other ultrasound (except a ladies' "internal" one!) I've ever had. It was pretty quick and I just had to turn around a bit so they could see each side.
Mine was done by one of the outsource companies, Care UK, as my GP was worried about some of kidney blood results and urine (blood, white cells, protein etc...)and he told me what he could see though you do tend to get more out of them, same as when I had my gallbladder scan.