Spacers aren't the most ideal solution, but sometimes there isn't much you can do. As above they are illegal..
I can see why they aren't really legal.. some of the universal ones are just rubbish. They move all over the place, have no decent wheel entering ring and are just dangerous as its near impossible to get the wheel and spacers 100% centered all of the time. Any slight looseness in the nuts will just throw everything out.
Saying that..
I do use quality spacers on my modern BMW using wider 8.5j wheels all round. I needed to as under hard cornering would only just rub the strut.. so used 5mm on the front, but due to widening the front and having a wider wheel than normal i then needed 10mm on the rear to get a more suitable track both front and back. I wouldn't go any more than 10mm.. and make sure you upgrade the studs to longer ones too. You want as much torque as you can get with the additional spacer between the wheel and the hub.
Make sure you get the spacers which are drilled exactly for your stud pattern, and NOT the universal ones. You will also need to make sure you get the correct centering ring. Some spacers include this.. others don't. This means you may need one for the hub to spacer, and then the spacer has the centering ring for the wheel. So can be a little tricky getting it exact.
Ideally they should look like this..
