Having used both lenses I'd say no it's not worth the upgrade.
The f/1.4 version is two thirds of a stop faster, which means where you'd use ISO 800 before you'd theoretically be able to use ISO 500. However that's only in the centre of the frame, the corners get significantly darker wide open. That sounds good, however I rarely use mine wide open so for me there's no advantage.
The 50 f/1.4 is soft wide open. All high aperture lenses are but this one especially so. It also suffers from bloom and longitudinal CA, defects that make the images unacceptable to me. I had heard all this before buying the lens and bought three copies originally (from different retailers, different batches). All performed the same so I don't think there's anything wrong with my lens.
It improves stopped down to f/1.8 and is good at f/2. Conventional wisdom says that you'd be better using the 50 f/1.4 stopped down than the 50 f/1.8 wide open. This is incorrect. Whilst the 50 f/1.4 does improve stopped down, it starts off so soft that it is only equal to the 50 f/1.8 at f/1.8! dpreview's lens tests actually indicate the the 50 f/1.8 is in fact slightly sharper wide open than the other lens stopped down.
Another reason to upgrade is build quality. Whilst the 50 f/1.4 is built better, it's still not as solid as Canon's other mid range lenses. The fact is the 50 f/1.4 doesn't have proper ring type ultrasonic focus motor, it has a micro USM which is no better really than a standard focus motor (though it is a little quieter). Full time manual focus is achieved by a special mechanism designed for the 50 f/1.4 and doesn't feel nearly as nice as the focus rings on genuine USM lenses.
Don't get me wrong, the 50 f/1.4 isn't a bad lens, I've taken many excellent image with it, it's just a pretty small improvement on the 1.8, and given the quality that lens produces compared to it's price tag, it's a bit of a marvel. That's why I'd recommend any Canon shooter to buy that lens even if they rarely shoot in low light. If Canon produced 50mm lens that was 3x better to justify the 3x price it would be one of the best lenses ever made for 35mm!
When compensating for focal length (to get the same field of view) a full frame 35 gives you a depth of field that is about 1.3 stops shallower, much more than the difference between f/1.8 and f/1.4
- In short, the 50 f/1.4 doesn't justify the price increase over the 1.8, the extra aperture range isn't that useful due to the softness, it's no better optically in the standard range (f/1.8 and up). Build is better but still not "solid".
So having said no I'll offer the following recommendation, get the Sigma 50 f/1.4. It's a more recent lens, built from the ground up to combat many of the problems with fast standard lenses.
- It has a bigger front element to combat vignetting (dark corners).
- It's sharper wide open
- It has a proper ultrasonic focus motor
I can't comment on build quality as I've never handled one, but as a bigger lens all round I would expect it to be a bit more solid. I don't shoot at 50mm enough to buy this in addition to the Canon but if anything happened to that lens I'd get the Sigma in a heartbeat!
edit: if you want to go down the shallow DOF / extreme blur subject isolation route then a bigger sensor may be a better investment than a wider aperture. Used 5D mkI are going fairly cheap these days and they make excellent portrait cameras. Here's an example of the difference, 30D on the left, 5D on the right:
P.S. I looked at your flickr stream - I really like what you're doing with the 50mm f/1.8, esp the "sunset" one! I wouldn't say you needed to have a shallower DOF in any of those shots.