Of course it had an impact. She was well on her way to an Obama 2008-type landslide before that memo.
There are a lot of people who still deny it had a sizable impact, and some liberals as well who just see it as Clinton trying to blame her loss on anyone or anything but herself
(which, tbf, she deserves some blame but it doesn't make Comey any less relevant). I just wanted to point out that both campaigns and other analysts agree that Comey did have an noticeable impact. For anyone who was closely following the election, it should have been obvious. I've never seen Politico's front page listings go from mixed news to literally 100% Clinton email stories within 24 hours. Every single link was something about the email scandal. They never did this for any scandal, not even Trump's Access Hollywood tape. This kind of beating in the media carried on for a while before election day.
I really wish Clinton had just retired - she had way too much baggage, particularly with the email stuff, but I do believe the last-minute Comey letter(s) shifted the election to Trump. Otherwise, Hillary was on her way to winning, probably by a pretty comfortable popular vote margin. Such a win would have probably flipped the Senate and maybe 6 - 8 more House seats as well.