This is where it gets tricky...for a half wrap of a van, it's upwards of £400, now, they can look lovely, however, a bit ostentatious for a window cleaner possibly?
My opinion, for what it's worth, is that if you can afford it, do it, but don't expect a huge return on investment. It's not going to gain you new customers down to the fact you have a spunky van, however, it will raise awareness of your brand, and consolidate the point you are a professional, and not a fly by night cowboy.
In terms of designs, big and bold works for me, not just a large image of water drops, but it's each to their own.
Go for it!