I don't see why people are passing judgement before even seeing any part of it. Wait until there's leaked footage or a trailer or something before passing judgement.
Live-action + CG doesn't automatically mean bad either, which a lot of people seem to think. It's actually the norm nowadays, especially when it comes to films that are based on popular franchises and our CG has come a long way now. After all, look at Transformers and most Sci-fi and fantasy films.
(Just so we're clear, I'm not necessarily referring to you guys, just people in general who are all "Oh noes, something new with Sonic in the title/on the cover! OH and it's live-action fused with CG! It MUST be bad!")
I have to be honest CG has grown up on some levels. Whether its self expression, or used in an action flick. Even in recent dramas its leveled the playing field on what visual engines are capable of. Note: Sanctuary, and Once Upon A Time are solid examples.
Lets put it this way. It will definitely look the nuts. CGI will be gorgeous and the cinematography will be ace (Sony will see to that).
But the issues here are in two things we currently have no knowledge about.
1 - Story. No matter how simple or complex, a story can be well told or not. We have no idea on story, and can only guess a Sonic vs Robotnik adversity based on the games.
2 - Acting. While direction comes heavily into this, the lynchpin of success will be in how believable it all is to the characters. Actors talking to green screen blue hedgehogs can, as seen in the fan movie, be very cheesy. The director has to be even more picky than usual and the actors have to have the talent to pull it all off.
I think the film should have just stuck with all computer graphics. I think of travesties like Smurfs.
Any popular movies that have appealed well to both kids and adults (that weren't PG-13 material) have been all computer graphics for the past 10 years. I'm not sure why Sony has to go against the grain and think that for some reason they are going to make the cg / live action mix work.