I have a G-Flex 212 powered by a Peavey Classic 50/50 power amp, and it will make my pants flap with the volume on 5.
The only drawback is that with the 212 sitting on the floor, it's aimed at my shins, so i need to stand on the opposite side of the room to really hear it well. My next purchase is going to be an amp stand of some sort.
I went through this debate a few months ago and was considering buying a Behringer UltraStack 412 stereo cab, but ultimately decided that a 412 was just too much to store and transport, given the frequency and size of the gigs my band plays. I considered a Mesa Rectifier 212 cab, but ultimately went for the stereo capability and ported enclosure of the Genz Benz, despite not having the opportunity to play one first.
A well-engineered 212 will push a ton of air and still be more portable than a 412. Look at Mesa, Genz Benz and Doyle for top-notch 212 cabs. I hear the Carvin Legacy 212 cabs aren't bad for the price, too.