I partially agree with the other opinions that you already have received. However, unless you are modifiying a car that 1) has a great deal of performance potential, 2) has a severely restricted air box/undersized air filter, you are not going to see the performance gains they advertize. I had a 95 Impala SS and that was a classic case of where an aftermarket filter worked wonders since the stock box was baffled and restriced to keep inductioin noise to a minimum. I changed the entire intake with a cold air induction and high flow cone filter. Changing only the filter media in the stock box would have defeated the purpose. On the top end, the modified engine really benefitted from the extra air flow.
K&N claims that as the filter gets dirtier, that it actually traps more dirt because the oil has attracted other particles of dirt that in essence become filter media. You decide. As for the MAFS problems, they can usually be tracked back to users to over lubricate the filter after cleaning.
I personally use them in all my vehicles, from the SS to the HD Crew Cab, to the E320, and the Triumph Daytona, partially for the performance/economy gains, but probably because I like piddling.
To sum it up, unless you have a car with some serious up side performance potential, or just like to piddle, you could spend the money else where. If you do use one, just make sure you follow the cleaning and oiling directions very carefully.