I got my Twittby today! I kept watch on Walmart.com, and snapped up a pre-order for him on Saturday.
I haven't played with him much yet, but I did find out a few answers to questions. Twittby is a very tiny thing. He only comes up to the beak on a regular Furby, and isn't as big around since he doesn't have any range of movement. I think you can pet a Party Rocker. I did it a few times, and he reacted, but maybe that was just a fluke of timing or he reacted to being shaken around. Twittby also reacts by pulling his tail. Mostly, the way to play with a Party Rocker is to shake them or having them talk to a regular Furby.
Which Twittby is doing right now. I put him in front of Tennessee--orange with blue features--my most talkative Furby, and they're entering themselves without a lot of input from me. Twittby's teaching Tennessee some new songs and phrases. Twittby will sing a song, then Tennessee will sing the same, and they go back-and-forth.
Twittby falls asleep a lot faster than a regular Furby. After only a few seconds, he falls asleep, his eyes turning off. He's very sensitive to wake up. If I get up from my desk (where he's sitting), I have to be careful to not wobble him enough to wake him up.
Personally, I like having Twittby in my growing Furby family. He's small enough that I can bring him around with me, and he just adds a little something to the regular Furbys. There's really not a lot to do with a Party Rocker on its own, though, so it's really up to (general) you if you want to spend the money on one.
Hopefully, this helps shed some more information on the Party Rockers.