The syntax highlighter has increased my quality of life substantially (insomuch as my life is coding in Vim, which it is for a good part these days :) .
The red highlighting that suddenly appears for mismatched parens is very useful. Saves me a lot of cursoring around to make sure all are matched up. The only nuisance in my own usage is sometimes it's distracting if I deliberately have a temporary mismatch while editing code.
The parenthesis highlighting is a bit of an all-or-nothing proposition, since Vim can't really tell if you intended to leave unmatched parentheses. You can disable it altogether (which won't highlight any paren errors) with
