When I was in college, having heard about the Dvorak layout, I decided to give it a shot. I switched my keyboard layout, applied some new letter stickers, and spent a couple weeks re-training myself to the new format. After about three months, I gave up and switched back, primarily for two reasons:
First, shortcut keys. The letter layout itself may be (arguably) more efficient, but the placement of shortcut keys is an overlay on top of that which has its own efficiently. Take Copy (Ctrl-C), Paste (Ctrl-V). They're right next to each other, and use the left hand so you can copy/paste while using the mouse with your right hand. If you use Dvorak, Ctrl-V is on the right-hand side of the keyboard, so you have to choose between moving your hand off the mouse, or using your left hand on the right side of the keyboard. I suppose you could re-map all your shortcut keys too, but that becomes an even more involved process with a higher learning curve.
Secondly, it became a struggle to use other computers. Although I'd retrained myself on my keyboard easily enough, it became more difficult to use other computers, and remember to switch back and forth. Hitting the wrong shortcut-key combination can have disastrous results in different applications, and it just became too difficult to deal with.
So, while the QWERTY layout may not be the most optimally efficient, in my opinion the overhead in switching it to anything else is simply too great.
It's still a great case study in how engineering decisions are made though, and I highly recommend giving it a try. Perhaps forcing a classroom of engineering students to do it for a quarter would prevent costly project overruns years down the road...