The 18" wheels which are standard in Germany and most of Europe have a shape that makes them easier to clean, which is one of the reasons I chose not to go for the 19" wheels (another being potholes which are less likely to destroy an 18" wheel).
I used Volkswagen rim cleaner (which I believe is the same as Porsche rim cleaner and I'm told is made by PS21 in Germany) which I had lying around and the brush that came with it and they again looked new with a few minutes work.
Keep in mind that I've had my car only for something like 3 weeks and 2,200 km (1,400 miles).
I use Griot's Garage wheel cleaner (not acidic like many others so very safe) and their long-handled wheel brush. Plus, cleaning them often keeps the excessive brake dust from getting too hard to get off.