JJ Abrams solves the biggest problem facing projects like this, prequels with a wealth of established mythology. Unfortunately, to avoid spoilers I can't tell you HOW he does it, but rest assured it works great.
The cast is just fine, conveying the spirit of the original actors without imitating them.
Incidentally I saw this in digital IMAX a couple of days after LASIK eye surgery. My doctor may have done too good a job: I was sitting in the 10th row and I frequently could make out the individual pixels. Technology - it solves problems and creates new ones at the same time.