I think intelligent life is pretty damn rare in the universe, bacteria and such are “more common”. But it may very well be possible.
That “they” frequently visit our planet however, sounds bogus to me. We like to think that they do because it sounds exciting and fascinating and we’ve all read about at least 10 whackjobs saying they were abducted and analprobe’d by grey aliens. There are a ton of movies and conspiracy theories about that stuff. Y’know, lizard people and shit. But I don’t think it’s real. There is no hard evidence at all, never was. Area 51 is a testing area for new military aircraft, of course it’s kept secret and unusually looking objects fly around there. That’s the damn point of it.
Anyway, because travelling faster than light is (as far as we know) not possible, it would have to be a big ship that can be maintained fairly easy and sustain supplies as food and spare parts for generations.
EDIT:
Forgot to elaborate on the self-proclaimed abductees/whackjobs. Attention whores, most of the people claiming to be abducted are one. Don’t believe it till it has really been proven. Though possible (alot of stuff is actually “possible” in some way or another), I believe it’s highly unlikely.