I used the paper bag way to make popcorn in microwave for awhile. Worked fine most the time, sometimes burned or didn't pop enough, or bag opened and got popcorn all over in the microwave.
I bought a Presto popper (amazon deal for $14)... pretty big difference, no burnt popcorn, almost all kernels pop nice and fluffy... I think it was worth it, little more effort than the paperbag way but popcorn comes out better. still cheaper in the long run than boxed microwave popcorn, and the chips and crackers the popcorn replaces for snacks.