There's not a lot you can say about a movie like "Hot Tub Time Machine." The only thing that really matters is whether it's funny, and I'm happy to say that HTTM passes that test with flying colors.
As for the plot, all you really need to know is this: "Back to the Future" meets "The Hangover." To delve into any more detail than that would be pointless, and probably spoil some of the best laughs. In an homage to the former, it's even got Crispin Glover, in a running gag about a bellman who either will or won't lose his arm during the course of the movie. It may be the most tasteless joke in the entire movie, but that's not likely to keep you from laughing.
John Cusack is good, and Craig Robinson from "The Office" is even better, but the real star is Rob Corddry as Lou, the somewhat unlikable loser who wants more than anyone else to have a "re-do" on the past.
"Hot Tub Time Machine" certainly won't go down in the annals of cinema as a great achievement. But it does achieve what it set out to do - and how many movies can, at the end of the day, say that?