I found it useful.

"I found it useful" is an acceptable sentence, probably short for "I found it to be useful". "I found it as useful." I believe is not acceptable to express the same meaning, but in a specific context it can express something different, namely a comparison: "I found it as useful as a car without wheels", expressing that something is completely useless. This type of comparison can be used with many adjectives, and always follows the same structure. Fx "as good as gold"