In the US, exit polls in the are a lot better than reputed, but unless you give caveats about them, reporters treat them as The God's Honest Truth—so maybe it's best if everyone thinks they're bad or else everyone goes overboard.
2018 midterms — mostly quite good
2016 general election — bad!
2016 primaries — mostly fairly good, caught a few surprises that polls missed
2014 midterms — quite good, caught GOP wave early
2012 primaries — up-and-down; tended to overrate Romney's vote
2008 — my memory is fuzzy, I recall them having some problems in the Dem. primaries but mostly being good in the general