I'm sure there are plenty of other reviews for this great lens, but I wanted to post my "everyday" person spin on it.
This is a great focal length on full frame bodies. I recently hurt my knee and have had difficulties (under exaggerated, torn quad) walking since September 1st. In the beginning of October, I brought my niece and her mom and sister out for senior photos, and this lens rarely left the body. Full portraits offered great isolation and clear focus consistently, and when I wanted to isolate her more for head shots, etc, the lens really shone. It's right in the sweet spot for my personal use.
By all means though, don't let this discourage crop body owners. I've used this lens on my 7D Mk I and while it's a bit tighter with the crop factor, crop sensors benefit from hitting the sweet spot of the lens, which can reduce imperfections based on perspective.
Color and vignette:
I am consistently pleased with the color and tone that this lens provides. The vignette is natural and actually at times pleasing. It can be noticeable in full frame bodies but I don't find it to be distracting.
In my experience, my 135L is one of my fastest and most consistent accurately focusing lenses. At times i am actually surprised how quickly this lens can grab and hold focus, making it quite capable for fast paced shooting like sports, etc.
The 135L has personally been my best bang for the buck lens, and I would be saying this even if I didn't get mine for an absolute steal (~$550 USD in cash/trade). Used in excellent condition, the lens retains a lot of value (pristine used copies can fetch upwards of $825 USD on the used market).
Seriously, if you have the means to shoot with this great lens, do it. You will not regret it. This lens is versatile for a prime lens and consistently sharp as a tack, while not breaking the bank doing so.