I've actually read some criticism of Colbert for playing essentially the same person, minus the fake conservative views and narcissism. But it's always possible that Colbert's real personality was a big part of his old fake character from The Colbert Report.
There have been some clunker bits and lousy interviews in the first couple of weeks - Elon Musk was painful to watch. But it's new, and on the whole, I'd say it's a better start than early Jimmy Fallon (on Late Night). If you recall, early Jay Leno was awful, and Letterman was winning the ratings war for awhile until Jay hit his stride. There are inevitable growing pains, and it takes some time.