Not as bad as seeing a trailer for a movie these days - they pretty much show you all there is to see in the main movie
MAkes going to see the film almost redundant
Here's my rule of thumb for trailers:
(a) is it a very long trailer?
(b) does it reveal lots of plot points?
(c) does it include scenes which are obviously from near the end of the film?
If any or all of the above apply, then it's (almost*) guaranteed to be a lousy film and you're seeing
all the best bits in the trailer. If (a) applies and it's a comedy, you've just seen
every funny line in the film. Don't waste 2 hours of your life.
(* There are occasional exceptions to the rule - it
might be a good film which the producers were too stupid to understand, so they don't know how to market it unless it has some big-name stars.)