How bad is it? It's average, or even a bit above average.
Should you see it? I don't see why not.
I have no idea why this shows up on worst-movie lists; it was actually the most popular film in England the year it came out. The humor is very outdated and sexist, but it's a great capsule of the time and place. Robin Askwith's butt is seen almost continuously, as he plays a window cleaner who services the horny stay-at-home wives in the suburbs. The idea that all women are oversexed (and reasonably attractive) is a common enough male fantasy. The humor works for the most part and the film chugs along nicely. Compared to most films I see, this is a classic.