Yesterday, I installed my aFe Pro Dry S air filters. The Touareg has two air boxes, so that doubles my cost for filters. Fortunately, with the aFe as with K&N, these are guaranteed for 1,000,000 miles. Because I'll be moving to a dryer, dustier climate this year, I opted for the dry filter over the oiled filters found in most aftermarket intakes.
According to the quick-reference sheet included with the filters, simply banging the filter against a solid surface will get them 86% clean, taking a vacuum to the panel will get them 97% clean, and washing them in a lightly-soaped water will get you 100%. Since these are dry filters, they are easier to clean and do not need to be "recharged" with an oil kit. They actually block 99.2% of dirt, as opposed to the 98.6% of an oiled filter.
These are a great option for any vehicle, but as there is 0% in the aftermarket for intake systems on my T-Reg, they are the only upgrade I can think of. I'm sure they'll afford me a few more horses since they allow better breathing for the engine, but fuel economy will also increase (I'm hoping) which is good since I've been averaging 12.68mpg over the last few tanks!