I was thrilled to see the Giana Sisters on a store shelf here in Australia about a week ago (really wasn't expecting it to be so prevalent here in Australia), so I snaffled it up. I got through it a few hours ago. Frigging GREAT game. I have fond memories of playing SMB on my cousins' nintendo back in the 80s, and this game is a pretty good match to that feeling. 
Where NSMB has diversified, Giana Sisters has appeared to have ratified and simplified and what you're left with is an incredibly simple premise, done very, very well. I would recommend it to any fan of the Genre.
But I have a question if anyone is able to help me: I have read in more than one place that (a) there is some way to unlock the original game and (b) some levels have alternate endings which lead to different levels.
I've played this game WAY too much in the last week. I got through it today, and once I find every red jewel in the last few levels of world 8, I will have as far as I can see completed EVERYTHING there is to do in the game. Is there any actual hard, first-hand information that confirms that the original game is unlockable, or that there are any more alternate levels out there on hiding? Or are these crazy rumours the result of some over optimistic speculation??
, but I really do think it's that good. It takes most of its cues from Mario (obviously, given its heritage), but with no run feature the physics are markedly different. Has big replay value, imo.