I played with Rosalina for one match (because my Best Buy sucks and disallowed 2) and won with her. It is really difficult to provide any accurate analysis because it was a FFA with items on, and most importantly, I (and everyone else, really) had a huge handicap with the Pro Controller with its awful button layout.
Rosalina is pretty strong and her recovery is AMAZING with a good double jump. Her only flaw would be her KO power, hurtbox frame and learning curve.
I wasn't able to use Luma effectively because of the FFA and such, but she's not useless without Luma. Her range increases quite a bit with Luma around. However, I think if Luma is tumbling mid-air, he won't attempt to recover and will fall to his doom. I'm not sure what ways there are to save him from this, because he is quite a sitting duck in this sort of position.
I can't say much about her KO power. I KO'd someone with a 1/3-charged forward smash at 100% and they were sent a bit far and were KO'd, so there's that. I heard from others she has a great Up Smash but I didn't use it so idk. Her Dsmash was weak iirc, but that was at very low %s so I can't say for sure. Her smashes will KO if you charge them up a bit, though, so if you are good enough to bait opponents you can get KOs that way.
Rosalina has good aerials. They have range and long hitboxes but they don't KO, I think (unless Bair does, didn't test this). Fair is a great way to approach. As an approach, Luma Shot is pretty good but has some lag, and can also KO if fully charged which is quite a nice surprise.
Rosalina definitely has a great recovery. She's able to recover from pretty much anywhere (unless below the stage, of course). She gains horizontal momentum from Launch Star and her aerial mobility + floatiness grants her to ability to move a good bit while helpless. She has a good double jump, too.
Rosalina's major flaw is her zoning. She has a high learning curve if you are to use Luma effectively, and without these skills, she can have trouble catching her opponents. Even if you use Luma as a shield, it won't work forever, against opponents with projectiles. You will have to be on constant duty to switch how far Luma is from Rosalina, while Rosalina does most of the attacking. Rosalina's smashes also don't have that great of range, so hopefully there can be other ways to KO. If Luma has KO smashes, it could be possible to use him as the finisher.
Also, I don't really know, but Rosalina's weight seems to be on the heavier side, like Samus, despite her floatiness. I didn't get KO'd until around ~120% but I honestly don't know.
This can all change in the final version but I do like how she plays even if the Pro Controller was awful. I'm eagerly waiting for the Wii U version to play as her more.