I really have no regrets with my cocktail cab. Neck strain doesn't seem to be a problem in either position (vertical/horizontal) and the major reason I decided on a cocktail was ease of display orientation. With the 3rd CP, you can play all games in their native orientation without having a rotating monitor, or black bars on the sides of the image. The only downside to the 3rd CP, as javeryh said, is it looks tacked on and can be a little akward.
The only regrets I have about the cocktail is the fact that driving games are pretty much a no-go, And shooting games, while do-able, will be weird because you're shooting down.
The only advice I can give is don't get a cocktail because of space concerns. I see this sited as the reason for a lot of people building/buying cocktails, but if you can fit a cocktail with a person on either end, you can fit an upright cabinet.
-Harry