I don't think I am going to say anything else that others have not already said, but maybe my perspective is a little different and my gear choice matches what you are looking for too. All my observations are based on using the 135L with my 5D mark I's.
Let's just cut to the chase: this is the best lens I own. Not just best in one aspect, but best in most. AF is very fast and accurate, just as fast as my 70-200 f4L (sold that lens to buy the 135L). Build quality is great. Feels very solid in hand - not heavy, but solid. Feels like it weighs about the same as my 24-105L. Bokeh is amazing, full stop. Contrast, saturation, and sharpness are all near the top of what Canon offers.
For portrait work, this is arguably one of your must haves. I know a lot of people compare this lens with the 70-200 f2.8L and there's good reason for that. I decided to skip the weight and bulk for the smaller and faster lens. Plus this lens has the edge if you shoot portraits since the smaller, black lens seems to blend in a little better than one of the bigger whites.
I use mine occasionally for car related events too. It's a good lens for focusing on details and interesting framing compared to the normal shots of whole cars.
This is also a great lens for around the house stuff like pets and casual headshots. I notice my animals are not as annoyed with the 135 as I can be a little further back and have more time to crouch down and compose my shot before they walk too close towards me to make the shot. A lot more doable than with the 50 or 85.
There are very few cons to this lens. Focal length is, ironically, one of them. This would be a tough lens to use on a crop I would think. It can have limited use indoors unless you have a good, clear space to work and your subject is fairly small (cats for instance are easy to shoot indoors with this, kids I would think would be too). Outdoors, this lens is more at home.
For portrait work, this lens comes every time along with my 35mm f2 (the old one). Sometimes the 85 makes it out too, but that lens has been a little neglected since picking up the 135L.
As mentioned above, I sold my 70-200 f4L IS for this lens. It was a really tough decision as I loved my 70-200, but I just wasn't using it enough to justify keeping it. I was shooting a lot of motorsports before, but that has dried up a bit in my area, so it sat in the bag too often. I sold my 7D (the 70-200 and that camera were a match made in heaven), so that left me shooting it only on full frame. I wasn't a fan of the focal range or using it for portrait work. Even tho it isn't exactly a monster lens, it was difficult to back in my shoulder bag with the 24-105. Finally, I made the tough call to make the change to the 135L and I couldn't be happier.
Some samples all with the 5D (these are hosted by my blog, you may want to check my Flickr for better quality examples):
[IMAGE'S LINK: https://flic.kr/p/t5rR3q]_MG_9161 by Bridgesphoto, on Flickr