The songs get better, and more fun certainly. The Boss battles (new feature!)vs. Tom Morello, Slash, and the Devil are kinda lame. On hard, for me anyway, the final battle to beat the devil is unpossible. So I looked online and everyone says one of two things:
1) You are ghey and a fag and should get better at GH3 n00b.
2) Wait til you get one special powerup, then deliver it to him at a certain time and you will win.
Either way, it kinda sucks. So far, he hands my ass to me because he has first shot at a powerup, then gives it to me at the worst possible time, which included my chance for a powerup, thereby sealing my fate because I can never crawl outta the hole he puts me in. Not much fun, but whatevs.
I used a cheat to unlock all the songs, because to unlock some like Dragonforce, you need to beat the game first, and I ain't gonna beat it against the Devil. So, all the songs are available to me and they are fun! Playing One, Number of the Beast, etc. is a blast, so GH3 succeeds in delivering some fun songs, but the tedium to unlock them legit is a little rough, but using the cheat somehow validates all the work so far and I look forward to rocking out some co-op!