Tried some of the songs in Rockband Network. The demo mode is a great idea, but just playing the first 30 seconds or minute of a song, I'm not sure how long it is, is very unsatisfying. I don't know what the rest of the song looks like! (thank god for youtube, and screw all the music companies that block the RB videos that I rely on to evaluate DLC!)
What they should do, is allow the whole song to be downloaded, but when you go to demo mode, it allows you to only play 30 second-1 minute sections of it. You pick the section, like in practice mode. That way you can see what the whole song looks like, but it is still crippled enough that if you really like it you'll buy it so that you can play the whole thing through uninterrupted.
And don't break the sections up into "boring repetitive part/crazy drum solo part." Have the sections mix it up so that we can try the transitions and get a better feel for the song, 30 seconds at a time.