I use a K&N in all of our Volvo's. I play with our cars, but not like a hobby...I'm serious about them. I want better gas mileage, better handling, etc. The K&N allows more air flow so it helps the car's power just a hair (not a huge amount like some people claim). However, the folks at K&N tend to "over-oil" their filters and because of this some people have complained that these filters messed up their expensive MAF sensor. If you go with one of these filters (which will save you money over the long haul), be ready to clean your MAF sensor within the first 1000 miles (really, an extra, preventative precaution)...then after that you'll be fine. When you re-oil it at the specified interval, only oil the side that is in the filter box and away from the MAF sensor and don't saturate the filter. I personally love the fact that I'll never have to buy another air filter and have not had a MAF sensor go bad from the oil in the filter.
'98 S70 T5 Emrld Grn Met/Beige Tons of Upgrades Mobil-1
'04 V70 2.5T Red/Taupe Some Upgrades Mobil-1
94 850 Sedan NA Drk Blue/Tan
'00 V40 Purple/Grey Mobil-1