I toyed for months over buying this lens. I was very happy with my 50 1.4, and was leery over purchasing this lens because of various focusing issues and mixed reviews I've read. One day, however, I decided to take the plunge and am so happy that I did.
Hands down, this lens is my favorite lens and is absolutely fantastic. I've now owned this lens for 8 months and it is on my camera 90% of the time (that title was perviously held by my 35 1.4L). Using this lens at 1.2 is amazing; it allows you to really hone in on a certain subject which makes the image really strong and powerful; not to mention, delicious creamy bokeh and super sharpness. I am a wedding and senior portrait photographer and when I use 1.2 for a single subject face close-up, eyes are super sharp and colorful. Most of the time I use the lens at either 2.0 or 2.2, and images at those apertures are super colorful and sharp (much more so than the 50 1.4). With this lens, images seem to come to life and look super sharp, yet creamy and smooth at the same time. It's definitely brought my portraits up to a new level, and playing with the 1.2 this lens has is really fun!
Construction is beautiful! It's solid and heavy, as every L lens should be. I once (gasp!) dropped this lens VERY hard on concrete while trying (and apparently failing) to do a quick lens change. It fell face down, without a filter, and I was devastated to pick it up and inspect the damage. I was SO happy and relieved to see it survived the fall without any damages to the external structure or internal function. Believe me, I did several focus tests to make sure the focus was still accurate and nothing was "bumped" out of place for the fall, but have not noticed any changes in focusing speed, color, or sharpness since the fall 7 months ago.
I have not experienced the infamous focusing issues reported on some other reviews. I'm not sure if it was just my luck with a good copy, but it has not had any focusing issues on my 5D or 5D MK II. Focus was tack-sharp straight out of the box, and have not experienced any focusing issues since. Focus zips into place both quickly, and quietly.
Overall, this is a fantastic lens and is worth every penny. Image quality and lens durability is more than worth the price. I only wish I would have bought this lens sooner!