is certainly worth seeing at least once 
(ROOT (VP (VBZ is) (ADVP (RB certainly)) (ADJP (JJ worth) (S (VP (VBG seeing) (ADVP (ADVP (IN at) (JJS least)) (RB once)))))))
(ROOT (VP (VBZ ) (ADVP (RB )) (ADJP (JJ ) (S (VP (VBG ) (ADVP (ADVP (IN ) (JJS )) (RB )))))))