I just did an entire studio photoshoot with this lens on my 5D3. I'd post pix, but they are NSFW.
The lens does make a little noise, but my hearing is poor and I stopped noticing after the first few shots. YMMV
I normally use the 50mm 1.2 L lens for full body and half body portraits at around F/1.4 to F/2, but recently I've decided I don't like the extreme sharpness difference between the model's eyes and the ummm... other body parts.
So I decided to try the 50mm F/1.8. I shot mostly around F/4 to F/8. Sharpness with the studio strobes was excellent. No flare either.
I really appreciated the lighter weight!
The main disadvantage is that the 1.8 looks small and cheap compared to the 1.2. If you do group photoshoots with other photographers, it might be worth it to have the 1.2. It also might be nice to have the 1.2 if you really love that dreamy blur, similar to what people exclaim about with the 85mm 1.2 L lens.
If you aren't working around other photographers, I'd say save your money and go with the 1.8!
BTW, I just posted some nice pix from the Arizona shootout on my blog.