First of all, for the record, Andy Sambergh's Marky Mark impression is 100% dead on, and I was genuinely impressed with with J***, but their magnum Opus is always going to be, to my mind, Lazy Sunday:
http://www.nbc.com/Saturday_Night_Live/video/clips/chronicles-of-narnia-lazy-sunday/2921/ (Sorry about the commercial, but I'd rather give you a high quality video link than give you a 'recorded from taping it off my hacked iPhone' link from youtube).
Musical Comedy is definitely one of the most hit/miss brand of comedy, second only to Non sequitur humor (Mitch Hedbergh = Good NS humor, Demtri Martin = Bad NS Humor), but Stephen Lynch is just good, even though he may be a dick in real life, and to be fair, Jack Black has said many times that Tenacious D started out as a just a musical comedy thing, before he realized that they were actually a competent power-rock/soft metal band.
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