Everything about the 135 f/2's image quality is "pretty", most especially the bokeh. I've tried 2 lenses with as good or better color palette, and one that I currently own (the Cosina Voigtlander 58mm) has better color. However, the contrast, longitudinal CA, and bokeh quality are quite a bit better on the 135 f/2 (and these count for a lot). Of course the focal lengths are different by a factor of two, so it's not really an even comparison. I suppose the color of the 135 is also more neutral than that of the 85 f/1.2L that I rented...it was too purple, and wide open the warm colors were also a tad too saturated. Both amazing lenses though. If Canon decide to stop making f/1.2 lenses, I will do my best to buy the 85 in the future, before it goes out of production.
If you are just trying to decide between a 100mm macro f/2.8, and the 135 f/2...they're really intended for different uses.
Wanting a 135 portrait lens, the decision is more between this and the 135/2.8 soft focus. I really like the fact that you can turn the soft focus on when you need it and off when you don't. It has three levels: 0, 1 and 2. The rendering is also nice, the bokeh is so smooth. Of course the f/2 is sharper, sharper wide open, sharper at f/2.8 and has less purple fringing... And just looks better, so it's not really much of a fight.
What you say is generally true, but I have an older 135/2.8 lens that flares and loses contrast worse than many wide angles! Of course it's from the 80's but it still proves a point Good lens design can produce great flare control and aberration control at any focal length. Try Samyang 16 f/2 or 14/2.8 on for size. Canon most likely will produce no more f/1.2 lenses. These lenses were made because the need for them was driven by the fact that you couldn't push your iso up to crazy numbers like 12800 or beyond. Now there's no need, most people seem happy using high ISOs rather than paying more for fast lenses and the people who are willing to spend the money on the fast lenses can't make it profitable enough for Canon (or any other manufacturer). Pretty soon the fastest lenses you can buy are f/2, then f/2.8... Unless something radical happens that makes it cheaper/easier to design and make good f/1.4 and f/1.2 lenses. We can always hope.
A glimpse of hope: Samyang is producing their first f/1.2 lens this year. Sigma might also start producing f/1.2 lenses if enough people want it, they're already competing for the money of Canon shooters (and I think Nikon too) and make excellent lenses, they just might take on the ef 85/1.2L and 50/1.2L next. We'll see! Then again, f/1.2 is less than half a stop faster than f/1.4 so there might not be enough of a reason to make more of the very special f/1.2 lenses. It depends on how many people are really willing to pay for them.
I guess the solution would be to just make an even faster, more specialized lens. Perhaps a zoom...an f/0.8 zoom would be nice. Or 2 or 3 of them. One that covers 28-50mm, one that's 55-110, and one that's 115-175mm (this longest one could be an f/1.0-f/1.6). The 5mm gap in the middle would be intentional, to help justify owning all three lenses. They could charge $4500 for the two shorter ones and $5500 for the longer one, all having IS and weather sealing. Most of the well healed canon bigshots would buy at least one of them, I suspect. Maybe not as many as would buy a super telephoto lens, but perhaps a good percentage of that number. Who knows?
Which lens is Samyang making at f/1.2? I may have seen this but I forget now. I'm considering purchasing their 14mm, because it is obviously excellent. However, I would really prefer 16 or 18mm, as 14mm is too extreme...has too much rectilinear projection distortion (I would wind up cropping and correcting...and losing resolution in the process). I considered the Tokina 16-28 as well. Not interested in the Canon wide zooms...unless they come out with a new one.
I disagree that few if any lenses will be faster than f/2.8 in the future, as you assert. I don't care how good the ISO performance is, there are still qualities the fast apertures give, that nothing else does. Besides, ISO performance is probably not going to be radically better than it is now, for the next decade or so...if ever...especially if all but pro DSLR's (such as the 1 series) somehow become extinct...replaced with a mirrorless phone mounted on the wrist or eyeglasses or something.
I would really enjoy those three lenses I suggest...