I got one from Costco for about £50 and it is lovely. It has a washable cover with a 'minky' side (furry/velvety) and a smooth one, and the blanket itself ties inside it to keep it in place.
I put it over me with a duvet on top, and can throw back the duvet if I am too hot, which is rarely, as I like to sleep in a warm bed in a cold room.
I have heard people say that only the expensive ones are worth buying, but I haven't found this to be the case at all. The Costco one is good quality and the weights don't bunch or fall to the bottom. I shake it out when I change the sheets, and that's it.
I think they only do one size and weight, though, and you need to be sure that they are 10-15% of your body weight. I think that that will be particularly important for children, as they might struggle to breathe under one that is too heavy for them.