A simple premise - Jack is about to get married and his friend Miles takes him on a holiday of wine-tasting and golf. Except the holiday Jack has in mind has a slightly different emphasis. And it's the last thing Miles needs right now...
There are enough reviews on here without me adding to the details of the film, but I recommend you stick to reading the positive reviews. The negative ones seem to miss the point, going by some of their ridiculous criticisms...
'the characters are unlikeable'.
Actually, the characters are refreshingly human and believable. They have their good and bad sides, like all of us, and the actors draw out the humour in their flaws, and even sympathy where appropriate. Especially Paul Giametti as the intellectual wine-expert, Miles, who was clearly once a popular, amusing guy until the collapse of his marriage sent him into a tailspin of uncontrollable alcohol binges. Anyone who dismisses the film because the characters are 'dislikable' I suggest either has a mighty opinion of themselves, or, more likely, sees something of themselves in the characters.
'It isn't funny'
We don't all have the same sense of humour do we? There isn't a comedy or comedian on earth that everyone likes. I found Sideways funny, but the comedy is consistently and delightfully subtle. It emerges from the characters, it's sprinkled throughout the dialogue and it's brought beautifully to life by the leads' performances. Yes, it probably goes over the head of anyone looking for easy laughs without paying much attention and those that dislike dialogue-driven films. What I will say is that I find it funnier with each repeated viewing (and I've watched it several times now!).
The film requires a modicum of patience and attention. If you can't be bothered to give it that then I would suggest that it is YOU who is boring.
The script is extraordinarily clever. It deservedly won an oscar and so too should have Giametti. His partnership with the oscar-nominated Haden-Church as the infantile actor, Jack, is a joy to watch unfold.
As I said, whether you find it funny depends on your sense of humour, but if you watch it with your brain out of gear and 'don't get it' then the fault is with you and not the film.