Size and weight rather than price alone pushed me to invest into the much lighter 55-300. From own Pentax forum tests, the AF tracking work better with the 55-300 screw drive than 60-250. The price, weight and size are all much better.
At 300mm f/8 is ok, good at 260 f/8, great at 190 f/4.5... You could almost consider it as a 70-200 f/4 with an option to go up to 300mm. A bit like a 18-135 is kind of a 17-70 with good quality and an option to go up to 135mm... People will complain that the long end is not perfect, but forget to formulate complaints that their 70-200 or 17-50 don't go up to 300mm or 135mm respectively.
I don't own the 60-250, my understanding is that at f/8 the 55-300 might be comparable to 60-250 @f/4, maybe not as good; And overall the 60-
250 will be noticably better for anything 250mm and up, especially for distant landscape/infinite were focus breathing don't come into play.
But for anything 250mm and bellow, I am not sure there that much of an incentive. The 60-250 much heavier, bigger, expensive.
The focus breathing make it doesn't have that much reach expect for quite distant objects. For birding you may find yourself at 200mm effective focal length or less. For near objects, the 50-135 provide actually more reach at 135mm.
For me as you only use that occasionnally, it is much easier to justify 300-400 €/$ than 800$/1200€ and in the bag, 450g/11cm much easier to justify than 800g/16cm.
You'll find that some people manage to use it successfully in Kenya or Tanzania for wildlife.
I started a thread on that here:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/315922-tamr...ya-safari.html
You find some interresting photos from FantasticMrFox forum member with that lens.
A few examples from his flickr account:
I'd be happy to get similar shot with any lens! And I hope so as I am going to Tanzania next september.