I use my copy (135mm f2 L) quite a lot. maybe not as much as my 16-35IIL, 35mm f1.4L or my 85mm f1.2 IIL. But it is used quite often. It works really well with extenders too, adding to it's versatility.
There's certainly room for improvement, my copy is looking a bit bashed up. It could do with weather sealing, although I've not had any issues with my copy and mine's got quite wet at times.
Mine's lost a screw on the front ring. I think there's certainly a capacity to push the design to f1.8, while pushing the filter size to 77mm. There's the option of adding an IS unit to the lens, as long as it doen't degrade the stunning Image Quality which the current version offers.
One of the joys of the current 135L is that it's a convenient size and weight. It's not too obtrusive and it's quite light and easy to handle.