I picked up this game without any prior knowledge of the Giana Sisters (growing up Commodore-disabled), just looking for a good platformer. I was not disappointed.
Everything is spot on. Your standard platforming goodness is there, but the twist is the method of changing between the sisters, which is brilliant. If you've ever played classics like Super Magnetic Neo (anyone?), you'll be right at home, as you'll be switching between 'worlds' quite frequently, and it makes for some fantastic gameplay.
Black Forest did a great job of defining the game mechanics, sticking with them (while adding variation), and getting just about everything they could get out of them. The fantastic level design encourages exploration, while the hard-but-fair gameplay rewards you for getting better -- which is something that not enough games seem to do these days </crotchety_old_gamer_rant>. The three bosses in this game will definitely cause screaming and controller-throwing, but when the smoke clears and you've beaten that final, crazy-hard boss, you will stand at the top of the mountain, gamer's high flowing through your veins, knowing that you did something worth doing.
In short, this is a great game!