I didn't realize I've had this lens for three years until B&H reminded me when I searched for this lens.
The bottom line: I would put this lens against any comparable offering from any other manufacturer.
I don't find the weight to be an issue (I'm 66 years old). I have lugged the lens on many cruises we have taken (Nassau, Haiti, Mexico, Alaska . . .) on either my K-1 or K-3II and I don't find it to be a problem.
It is fast focusing, and I find it to be sharp as a tack at 450mm when you're in it's sweet spot of under 20 feet or so. While I have the 1.4xRC, I have not used it with this lens. so I can't talk about that.
Unfortunately, B&H is limiting me to only two pictures. Highly recommended . . .

Jerry

I have no complaints about this lens,it is sharp, sharper than the Sigma 150mm-500mm I was using. Very solid build. It performs excellent throghout the entire range,even with the Pentax 1.4 teleconverter

Its quick and very sharp at 450mm. Colors are nice, the bokeh is good and its built solid. The focus and zoom are smooth. Even though big and heavy I think I made a good purchase for a long zoom.

Read other's reviews before plunking down the money. No doubt, it's heavy! In use with a Pentax K-3, beautiful images, works great. Combined with a 2X Teleconverter for up to 900mm focal length it's better than a telescope! However, AF doesn't work with the Vivitar 2X TC but the rig will be for tripod photos only.

Yes, even when hand held . I waited to respond until I had a good opportunity to use the lens this first day of Spring 2019. I walked around our local lake carrying this heavy lens and shot turtles on logs, mallards, cormorants, mud hens, and an osprey. I shot handheld, wide open with a fast shutter speeds and the ISO set between 400 and 800 using a Pentax K1 Mark II camera body. The images are sharp with excellent contrast. A great lens and I look forward in capturing more wonderful images. Thank you Pentax for another well built quality lens!

Yes, it is heavy but to be expected for the size and quality.� Focusing is great on both auto and manual.� Mine didn't come with a manual but I did found one on-line so I can make presets, etc.� Recently shot an owl at dusk and got some great images with it. Here is one with heavy cropping to show the owl's face.� Thought it looked great at 800 ISO with all the cropping...

Sharp, fast focus, great color. Focus controls on lens are well placed, the focus limiter is a big advantage. Solid build so should give years of use, will likely outlast me. Expensive but the cost, like the weight, are part of the package.

I am using the lens for wildlife and some scenic photos with Pentax K3 and K1 cameras. I purchased the lens because my Sigma 150-500 lens does not work the the full frame K1. The Pentax lens is sharper and auto focus is much faster than the Sigma lens. I very recently used the lens in light rain and the lens performed very well. It is worth the cost. The lens is fairly heavy so I recommend using a tripod.

Everyone mentions that it heavy

Coupled with the K3 the focus is fast and accurate.

It takes great pictures.

Hi, everyone. I bought this lenses when I see the price tag at B&H without much regard in summer 2015. I think I have a new learning experience on how to handle the lenses first. This lenses weigh 3 times of my K50 and KR, MZ-3, 2 Pentax LX and handle with care with the hood on. Image quality is very high with little softness at the corner. I have not bring the lenses out in rain and snow day yet, but winter is coming and I may test it. The button on the lenses is something new to learn about since this is the only Pentax Ienses I have had buttons to command the lenses to do different thing. I recommend to practice at the wide end (150-200) first before slowly move on to longer end. One can easily lose out on some shot, if one do not know where the button is or how how to switch focus mode. On my LX, the lenses would bring me bring me back to near Stone Age with some civilization. Since the LX only work in stop down mode(no silence AF, no shake reduction, no auto exposure). But one thing remain is the weather resistance feature of the lenses will fully work with Pentax LX. Well, I will find out someday when the rain come. I really like to use this lenses on all my camera ( K-50, KR, MZ-3 2LX )