Although they are both technically outstanding lenses, I never warmed to the 40 and 70 limiteds either. Having a FA43 and FA77 in my bag probably didn't help.
The DA*50-135 is however one of my all-time favourites; it is very sharp, it renders colours and out of focus areas beautifully and the handling is superb, not to mention the weather resistance. It is much smaller than a 70-200/2.8.
I had a DA*60-250 for a while as well, but it felt much bigger, was a stop slower, and it became physically very long when fully extended. It also does not actually give much more reach than the DA*50-135 if the subject is not a long way away. On top of all that, I preferred the images from my DA*50-135.
And I completely agree with the OP about the 15 and 21