According to Dictionary.com, it's pronounced either "fun-guy" or "fun-jai" ("guy" with a "J" sound).
"Fun-jee" is just plain incorrect. ..And it really bugs me when people pronounce it that way.
Fungus is pronounced with "gus" not "jus," so the plural form sounds better with the "G" sound, IMO, as opposed to "J." So, I say it as "fun-guy."