I found Looney Tunes: Back in Action to be unfortunately quite disappointing. Joe Dante is a great director (Gremlins, The 'burbs, etc.) and this is right up his alley, but the movie just wasn't as good at mixing live action and animation as Who Framed Roger Rabbit. I went with seven people to see this movie and we all came out very disappointed - I probably liked it the most. Everyone else actually hated it. Steve Martin (one of my favorite comedians) was actually annoying in this movie, which is unusual.
Good review though - it's nice to read another perspective.