This is pretty much Planes, Trains, and Automobiles for a new generation, except this film is quite as funny or well-made. It's not a remake of that film, so much as it is a take on that film's general concept. With this one though, just swap out Thanksgiving for the birth of a child, and take away the trains (well, there are cameos, but that doesn't quite count).
Downey Jr is the straight man, Galifianakis is the obnoxious (big emphasis there) but well meaning (we hope) comic relief, and joining them are a number of nice little cameos, with one of the best being from Juliette Lewis, although Foxx is not bad. I like Michelle Monaghan, but could have used more of her. I feel that way about her career in general, though.
There are some laughs, but I never found this film to really be hilarious. There are some serious, dramatic, and even legitimately touching moments, but overall, tonally this film is inconsistent. PTA was a little mean spirited besides heart warming and hilarious, but it also felt more convincing, and the changes in tone and character were earned. This film doesn't quite get there. It sure tries though.
I think the main thing is that this film had a lot of potential, but, to tie in with the film's plot, it's a bit premature. The script could have been retooled and some of the story elements could have been reworked to get the audience to care just a tad more, and have it stick.
I did like the performances though, because those were decent, although Galifianakis did get on my nerves, which doesn't really ever happen. Maybe I'm just tiring of his shtick. Some of the film's big moments are cool ,but some are maybe just a little too over the top or ludicrous.
All in all, I can't bring myself to give this a mild recommendation, even though I didn't find the viewing experience of it to be unenjoyable. I actually do sort of recommend it though, like mostly if it's on cable or something. Otherwise, it's not really a film that must be seen.