I think the curve up/down good/bad depends a lot on the people involved. My wife has a nicely sensitive g-spot, so doesn't require a lot of curve for stimulation. However, a little more curve up is a good thing to her. On the flip side, we knew a couple where they guy had a pronounced downward curve and she just couldn't enjoy him, regardless of position oddly enough. Yeah, you'd think it was his technique, but she said it was "all about the downward curve" that made his penis uncomfortable for her. Not sure why reverse cowgirl didn't solve the issue, but it didn't.