School Days is... something else that's for sure.
The funny and or vaguely interesting point there is that it's again based off a VN and pretty much went with one of the worst possible ending paths. If I remember right, there are a handful of bad endings in a similar vein but the anime went with one of the darker ones. Admittedly that's exactly what sprung to mind too when you said that, Liam. 
Yeah, it was the type of thing where I asked myself why I watched it after. Its definitely something, as you said. As for the VN to anime adaptation, they pretty took the worst endings (which are actually hard to get in the VN) and combined them for lolz.
I dunno, sometimes adaptions are pretty good but since it gets brought up a ton anyway, the When They Cry adaptions are supposedly kind of... bad once you compare them against the source material. I can't comment there since I haven't watched either season proper and only read a chunk of Umineko but it sounds like they cut a lot out (both the question and answer phases of both sets are very long).
One of the frustrating things about my main computer being a Mac is that I rarely get to enjoy VNs unless I jump through a bunch of hoops.