Ok so I rented GH: Aerosmith honestly to play only two songs on it (Dream On and Walk This Way), and I can say I'm glad I rented it. First off, the difficulty is really easy compared to both GH2 and 3. I'm on the 4th set (out of 6) on hard and haven't gotten anything lower than 5 stars. The achievements are LOLable (seriously one achievement is play a song as Izzy Sparks WTF?), and honestly it's boring to play.

Here's a detailed Pros / Cons area

Pros
Achievements are REALLY easy (like back in GH2 days)
Career difficulty achievements are stackable now

Cons
Easily gets boring to play
Achievements are a joke
All the outfits and styles for original characters seem like a waste of money since you mostly play as Aerosmith

Official statement - Rent for an easy 700+ gamerscore added to your gaercard

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PWN THEM!!!