SS,
I have this lens and purchased it early on in my journey into DSLR photography (2 years ago). My intent was to use it mainly for the same reasons you are, wildlife and landscapes, to coincide with a trip I was taking out to Yosemite. The main impetus behind the purchase was that it's a convenient package with a lot of bandwidth in a single lens, simplifying what I had to carry around the park and eliminating my need to change lenses. I was using it on a D7000 and while I was happy with the shots, the truth is that I could have used something a little wider for the vistas and was glad I had my 18-105mm along. Had I been shooting with a D600 (which I have now) it wouldn't have been an issue since 28mm on the FX body is about the equivalent to the 18mm on the DX.
OK, now as for the lens, my take is that it does a great job for a zoom that size and price, and that covers that much ground. At the extremes, and the high extreme in particular (300mm - which is actually closer to 270mm), it's not as sharp as I'd love it to be, but it's definitely within the realm of what I'd consider "acceptable". From 50-200mm it's much better (not tack sharp, but definitely sharp) provided you're not wide open where it reverts back to acceptable. Honestly, I didn't perceive and lack of clarity until after getting some additional lenses after shooting with this one for about 10 months and seeing the difference. If I have a beef with the lens is that in resting mode (lens pointed down) it's going to creep rather quickly to full extension every time, so I have to lock it at 28mm walking around. No big deal, my 150-500mm does it too.
When I started shooting I concentrated on birds and animals in the wild and while this did OK I found that I needed more reach and got a Sigma 150-500mm that is now my predominant wildlife lens (sharpness is about the same across the range). Likewise, I wasn't that into landscapes at first, but now it comprises a lot of what I shoot, so I'm finding that more times than not I am shooting to one extreme or the other with this lens and it's not used that much (the low end has been taken over by the 24-85mm kit lens I got with my D600 and an 8-16mm Sigma DX lens that I will still use on my D600). The one thing I find myself using it for now is for concerts and music festivals where one body/one lens is all I want to worry about keeping track of. I'm seriously considering selling mine, so if you're interested pop me a PM and we can talk about making a deal.
Here's a timeline of my experience with the lens, starting with my earliest pics which suffer more from my lack of experience with the software and RAW imaging than the lens, to more recent stuff, just so you can get an idea of what it's capable of. As you can see, the more I learned, the sharper the photos got. Hope this helps.
