It is "of much importance".
or you could also say.
"was very important to us."
2010년 6월 11일
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"His advice was of much importance to us."
2010년 6월 11일
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Was much importanT;
So is of + noun <<< is right, see
http://dict.youdao.com/search?le=eng&q=be+of&tab=&keyfrom=dict.top
But you should note that not every noun can be used this way, roughly the abstract nouns can. Like 'be of value', 'be of help', 'be of significance' etc. So 'be of noun = be adjective (of the noun)' eg importance vs important.