The new Guitar Hero has a great selection of songs for the people who love metal and rock. This is a more hardcore game than the two predecessors. The track selection allows for great gameplay with addictive songs and improved note charts, but people who do not love metal or hard rock will not find a lot to like.
The new Quest mode is fun but far from revolutionary and the first playthrough will take you probably less than 4 hours. Quickplay+ gives lots of replayability, but will again be mainly attractive to the dedicated since it's quite complex and caters primarily to those that love leaderboards.
Back to the roots indeed, but at the same time this strategy has not brought any real innovation. Also the production values are fine, but nothing that comes close to The Beatles: Rock Band, something that would be expected seen the lack of other new things to enjoy.
With Rock Band 3 around the corner, it's enough for the dedicated fans but otherwise would recommend waiting for RB3.