I have the backpack model from Peaceful Valley Farm Supply, and have used it quite a bit. Its pricey, but saves my back from lugging around a propane tank. One pass over young, annual broadleaf weeds is all it takes. You don't need to "cook" them, just boil the juices. The expansion of the liquid in the cells bursts the cells walls. The weeds should just look a little darker after you pass over and then wilt within an hour. If you "burn" them you are waisting propane and quite likely killing more good bugs than necessary. Killing good bugs is a consideration with a flamer, as is the very real possibility of burning crops if you get too close, and the difficulty in killing perennials and grasses. The flamer is worthless on my nemesis - quackgrass. And, as has been mentioned, when you kill off everything else, the really bad weeds then have no competition and have a party. I also can't use mine at all after I lay down my drip irrigation system in May, so the flamer spends its life in the shed these days. I have better success using cover crops to smother out weeds. But, the flamer is very useful for some applications. It depends on you weeds and your set up.
