They are difficult indeed near impossible with the factory supplied tool and hammer, which are useless. I'm sure the Mulfab tool will be a revelation, however it's too big to take in the car, I think. I have a tool from MWS who supply the wheels, it's a brass two eared tool that fits properly and I use a bigger Thor nylon hammer to whack it. It's easy to carry these in the car.
http://shop.mwsint.com/productslist.asp?catid=128&pagenum=1Second item on the page, KAS42 cost £12
Getting them off is more difficult than putting them on. When putting on, you don't need to tighten hard because the rotational motion of the car tightens the further.
Some people like two eared spinners but you need yet another tool for these!
Good luck
Nigel