I would be surprised if there isn't or wasn't at some point intelligent life in the Universe. Thing is its a big place. What I find almost impossible to believe is that these aliens have solved the many many problems to reach us like the ability to travel near or past the speed of light, the energy requirements to travel at those speeds, the radiation problems with travelling at those speeds and then either crash when they arrive here or allow themselves to be seen when they are trying not to be seen, which I would presume they are doing as they've not announced themselves. So yes if someone says they saw aliens I would assume they were lying, nuts or I will add mistaken, though the fact they went with aliens as an explanation says a lot.
If someone said to you they saw Jesus or Zeus or Shiva would you think, oh maybe they did lets listen to their evidence?