Let's get this right out of the way, this is not a science fiction movie, time travel is just treated as a magical effect that lets our heroes travel back in time, where they do all kinds of stuff that should create any number of paradoxes.
The premise is: Three friends and a nephew take a trip to relive the friends' glory days, staying in the same hotel room that they stayed in 24 years prior. After a night of drinking heavily in the hot tub, they dip a Russian energy drink (it's got illegal ingredients!) into the timing control of the tub, and wake up in 1986. They appear as the same to themselves and each other, but reflections in the mirror, and other people perceive them as their 1986 counterparts. Except for the nephew, who wasn't born yet in 1986.
What follows is an interesting look at three people, whose lives haven't been going where they thought they would, view living through their teenage glory day again. The humor is good, if more than a little vulgar.
I had a fun time with the movie, but the ending did make me think that maybe everything wasn't as hunkydory as it could have been.
The movie ends with each of the three friends making the starts of very positive changes in their lives, and repairing the hot tub, so they can return to their own time, at the last minute, one of them decides to stay in the past, and live his life over again. The remaining 2 friends and nephew return to the present, where the friend who stayed behind has left the bell boy with instructions on where they now live. It's revealed that he's exploited his knowledge of the future to make himself filthy stinking rich, most notably, Google is now Lougle.
Cusack's character finds himself living with the intelligent funny girl that he met on his second time around at the hotel, and the last man has become an award winning music producer.
At first glance, everything seems fabulous for these people. But the three people that rode the time machine forward no longer know anything about their lives. They are 24 years in the dark, they may have great lives, but they don't have any of the memories associated with getting there. Instead, they've got the 24 years of living their mediocre lives from a timeline that no longer exists. Sure, this will definitely help them better appreciate what they have, but they miss out on all of the wonderful experiences that lead up to it.
Not to mention all of the birthdays, anniversaries, job skills, and just other factual knowledge that they're going to be lacking.
You could argue, since it's more magical than technical, that maybe they'll eventually discover all of the memories and knowledge that they've acquired in this new timeline, which I'm inclined to do, since I love a happy ending. But it certainly leaves it open to question.
Bottom line, this is kind of a cynical redux of an '80s teen comedy, not a science fiction comedy a la Frequently Asked Questions About Time Travel. But it's still a lot of fun.