It is "of much importance". or you could also say. "was very important to us."

"His advice was of much importance to us."

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.