Well, I'm back. I only got to hear it once, so I really can't be too specific. I think it's longer than the demo, and her vocals sound really good on it. It sounds like Britney song, not Ke$ha. There are some really nice harmonies on the chorus...I'm not sure if they're done by her though. Very 'danceable', nice production, different to the demo I think. It's not anything ground-breaking, or 'TOXIC 2.0', but it's a good, very melodic lead single. It was more instant than Womanizer for me. I think there are some lyric changes as well, I can't say because I haven't listened to the demo that much.
Toxic isn't my favorite song by any means, but it's very innovative and it could have come out 10 years from NOW and still been a hit. It's just such a classic.
But anyways, I'm not here for another Toxic. I want Hold It Against Me 1.0.
GOES TO TWEET IT! 
