There are SOME really good replicas, the lambo ones tend to be the best, there are a few good Ferrari ones but those ones based on MR2s just don't look right because they're not long enough. You can tell them a mile off.
But I saw a Lambo one for sale not long ago and it was incredible, you honestly wouldn't have been able to tell the difference, by looks, quality of build or sound. It was nearly £40k though and sold very quickly.