It is "of much importance".
or you could also say.
"was very important to us."
١١ يونيو ٢٠١٠
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"His advice was of much importance to us."
١١ يونيو ٢٠١٠
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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.