I enjoyed 'Four Christmases', even if it does gradually drop in quality throughout.
I found the premise and set-up of it entertaining, the execution may not be perfect but it leads to a satisfactory Xmas comedy. The plot does get slightly repetitive, but there's enough in there to keep it watchable. They also, smartly, keep the run time low at 88 minutes.
Vince Vaughn and Reese Witherspoon are a fun pairing, I felt the necessary chemistry between the two. Elsewhere, the likes of Robert Duvall, Jon Voight, Mary Steenburgen and Jon Favreau all make noteworthy appearances.
It's good. If it kept up the strong start then I'd be rating it a tad higher.