Except that strategy has the known drawback of producing lower revenues and higher trouble ticket counts. So I guess it depends on your goals.

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Depends on how careful you are producing your "known bad" list, and heavily depends on the user. If, for example, Amazon ever "degraded" the page they send me because I'm using a new-ish Gecko or WebKit browser that isn't in the approved UA list, you can bet that'd be a lost sale for Amazon.

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The existence of good, open-source rendering engines has basically turned this whole thing on its head; years ago it was better to assume that a new browser was deficient. But today, on average, it's better to assume that a new browser is embedding someone else's engine and is thus as capable as the well-known browsers.

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(all of this, of course, ignores larger issues like progressive enhancement and graceful degradation, which are just as important and make UA sniffing even more irrelevant)