Quoted from EalaDubhSidhe:

It's about how impact angles affect momentum. Straight ramps at those crucial few inches bottom guarantee that any point the ball hits on the way up will be at the same angle relative to the shot angle from the flipper. What you need to do then is ensure that the two angles are not too far apart from each other. It doesn't automatically make the straight ramp *better* than a curved, but it does make it a damn sight easier to design properly. Solidworks in particular demands straight lines on easily defined gradients if it's not going to give you a massively finicky time about it. Certain types of ramps, such as drop ones, also have to be straight at the bottom or they just won't function. But curved ramps are AWESOME as long as they're done right, and the ball doesn't clonk against the wall when it's shot in; the ball also hugs the curve more because it's automatically channelled that way. As long as the curve is sensibly shallow with a wide radius, and the entry angle cross-sectioned between the two sides is close to perpendicular with the ball shot, you should be alright.