Dear WO'N: it's so useful, it is very widely used. My dictionary seems to accept it:
2 a) to the degree expressed or understood; to such an extent !why are you so late?" b) to an unspecified but limited degree, amount, number, etc. !to go so far and no further" c) to a very high degree; very !they are so happy" d) [Colloq.] very much !she so wants to go"
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