There's some good info here, but I'll disagree on the general idea that fast lenses aren't very usable wide open. Just because you don't shoot very often below 2.8 does not mean that no one else does. It's more about your personal shooting style and the conditions you shoot in (f/4 indoors is a bad situation unless you've got great light or are providing your own). Specifically, for my 3 50mm lenses, here's how often I've used the apertures below:
50mm 1.8 - 48% of all shots at 1.8, 74% at 2.5 or below
50mm 1.4 - 30% of all shots at 1.4, 50% at f/2 or below
50mm 1.2 - 52% of all shots at 1.2, 82% at f/2 or below
These are only shots that were kept, not shots where focus missed or only someone's nose is in focus.
Based on the numbers, it looks like we have very different shooting styles, however many of my shots are at 2.8 - they're just taken with different lenses. For indoor/low light shooting, though, these fast lenses are great to have. If my zooms were faster than 2.8, I would likely be using wider apertures with them as well. I'm not sure if you've had a chance to use a lens like an 85 1.2 (or the Sigma 85 1.4), but there's a reason that people pay a premium for fast lenses.
If you're considering this lens, take a look at some samples on Flickr to see if you like the look/DoF @ 1.4:
50mm @ 1.4 on a 5D II: http://www.flickr.com/search/?q=p...=50&xfh=50
50mm @ 1.4 on a 7D: http://www.flickr.com/search/?q=p...=50&xfh=50
Not to mention, the wider aperture is not the main advantage of this lens over the 1.8. You get better/faster AF, a usable focus ring, full time manual focus, and a huge jump in build quality. If none of these matter, then you can evaluate it on the optical qualities alone.
As for waiting for the non-existent 50mm 1.4 II, as you mentioned, it will be significantly more expensive than this lens. Also, at this price, you can buy this lens, use it until the new one is out, then sell it for the price paid (or close). I agree that the 50 1.8 is the best bang for the buck from a price/performance perspective, but this lens provides some significant upgrades and shouldn't be overlooked at this price.