Pretty fine in my opinion, I can get them all cleaned up - as well as the brakes and tires in 5-10 minutes.
Although I must say I like them with a little dirt on as it dulls the alloy a little and blends it in with the rest of the car... When they're clean they are a little too bright silver for my taste for the black car - they look ace and really pop, but I like them a little duller from a few days driving ;p
I cleaned mine yesterday using autoglym custom wheel cleaner. Bearing in mind I have only covered less than 250 miles in the 2 weeks I have had the car and cleaned them twice it was fairly easy. After gently agitating the cleaner with one of those hedgehog brushes then light pressure washing holding the jet a decent distance away they came up ok. Finally used left over shampoo to go over the wheels again. Seems like a lot of work but to be honest you only have to spend the time on the front wheels because of the brake pad dust.
I have used Astonish wheel cleaner in the past 97p from Wilkos on my old Polo and to be honest it did a decent job using the same method. I don't think it is acid based but the key is to wash it off immediately just like the autoglym.
Just remember the brake calipers are visible so be careful not to over spray any cleaner or direct the jet wash with a powerful jet at them as with the tyres.
I hope I have been helpful.
When we had our milkround we ran a couple of electric milkfloats with large alloy decks and I found biological washing powder was a very good way to get & keep the alloy clean & bright. Not sure how abrasive this would be on wheels though!
I've just gone with car shampoo on the Lupo but the alloy wheels are quite smooth and have an open spoke design which is easy to clean!
Little and often seems to be the best way to keep them in good trim & don't let the brake dust build up too much.
hot soapy water with a decent sponge, run the sponge into the inner rim, then the vw centre cap then the inners of the spokes. I think the key to nice looking wheels isto cleanthem on a regulare basis.
Be careful with wheel cleaner chemicals - these can sometimes cause damage to the wheel especially if not used correctly. As others have mentioned, don't let the brake dust build up too much, sponge with car shampoo (separate sponge for the wheels), rinse with clean water and dry with drying towel. If build up is a bit much the Autoglym wheel cleaner is good but would not use any every time.