It's Guitar Hero 3.5...kinda
Now how is it like Guitar Hero 3.5? Well even though the menus, loading screens, and backgrounds are really the same ones from Guitar Hero 3 but with Aerosmith stuff slapped on, Activision made some improvements to a few of the problem areas of Guitar Hero 3. For example, Calibrating the Controller with an HD T.v. has always been a pain in the ass. In Aerosmith the calibration is much easier, this time you're able to calibrate the audio, and video either together or separate. Another improvement is that now for songs that were done by cover bands, credit is given to them as well.
Probably the best change made is with the Pro-Face off mode in multiplayer. Now both players may choose to both play on different difficulties. When they face off the points are scaled to it is possible for one player playing on Expert to lose to someone on Hard. While this may anger some vets of Guitar Hero it now gives a chance to those of us who are still stuck playing medium on some of the more difficult songs... Which brings me to the song difficulty.
The songs in Aerosmith are much easier then the ones of the last set in Guitar Hero 3. While they will give a good challenge to some, those who can play Through the Fire and the Flames on expert will probably get bored very quickly, or may not, depends how much they like Aerosmith.
In the end, despite all the minor improvements, Aerosmith is still just Guitar Hero at it's core, with over 40 new songs to play while we wait for Guitar Hero World Tour this fall.