These are really easy to clean. Put some warm/hot water and soap in and shake it. If you let something sit in it, you may need to let it soak in hot water for a bit to losen it up some.
I think they mix the powders well. If you are making a larger shake, I'd use a bigger shaker bottle, because it does work better if you have more shaking room. So if you get a smaller bottle and fill it up, it may not mix as well. Sometimes it may take a little while to shake it well enough to thoroughly mix it, but to me, that's expected as you aren't using something electric to mix with.