It's just a M4 mesh slapped onto instead of default model. Still has same animations so it doesn't really follows the same feel as original.
Sounds are replaced, but sound cache is not supplied, resulting in broken ingame sounds. Author either needs to generate one, or strip replacement wavs to match same length. There are L4D2 sounds modding guides, they are applicable for L4D1 too.
Model looks fine, because Valve made it and they know how to properly make for Source engine. Atmosphere wise, it's fittable to L4D.