They're saying: Uneven at times but emotionally-charged and impacting.

Most acting praise for Joseph Gordon-Levitt.

http://www.variety.com/review/VE1117946047?refcatid=31
Will Reiser's semiautobiographical script initially prescribes too artificial a story treatment for its characters but is rescued by a genial, low-key vibe that builds in sensitivity and emotion up through the final reels.

http://www.hollywoodreporter.com/review/5050-toronto-film-review-231741
Strangely, the movie achieves its greatest success in its third act, the place where most movies collapse. Perhaps because the seriousness of Adam’s condition is more apparent by then, it’s able to better convey the dark comedy promised earlier. The emotional balance is more stable and everything feels more real and less … well, sitcom-ish.

http://www.avclub.com/articles/tiff-11-day-0,61477/
Some of it feels a little buffed-out and commercially minded to read as real—and the film errs severely by depicting the protagonist’s reluctant girlfriend (Bryce Dallas Howard) as two-timing and villainous, rather than merely overwhelmed by a commitment she can’t make—but damned if it doesn’t get the laughter-through-tears effect it wants anyway.

http://incontention.com/2011/08/18/5050-draws-laughs-and-tears-but-gordon-levitt-owns-it/
This is Gordon-Levitt’s show. He gives is best portrayal to date here, mining rich emotional beats when they count and capably driving forward a character that could have come off a bit cliched. I was particularly moved by his display when heading off to a scary procedure late in the film and couldn’t keep the tear ducts dry there to save my life. He really finds something special in the emotionally charged areas of this performance.

http://collider.com/5050-review-seth-rogen/113412/
There’s something sweetly genuine about the way the characters use comedy as a self-defense mechanism against a terminal illness and it probably comes closer to capturing how those suffering from the disease actually react than any melodrama.

http://hollywood-elsewhere.com/2011/09/5050.php
Jonathan Levine's 50/50 is an exceptionally honest, no-punches-pulled, very honorably acted adult drama about a young guy (Joseph Gordon Levitt) grappling with The Big C. It's seasoned with occasional laughs, for sure, but there's no way this is a light mood comedy, as the 50/50 trailers have implied. And I mean that with the utmost respect.

http://blogs.indiewire.com/theplaylist/archives/tiff_11_review_the_admirable_low-key_50_50_splits_the_difference_between_ge/
In hopes of making a balanced and fully dimensional, living, breathing movie, the congenial and even-handed picture ends up not particularly funny or especially effective in its hoping-to-be-quietly-devastating moments.

http://www.hitfix.com/blogs/motion-captured/posts/review-seth-rogen-and-joseph-gordon-levitt-star-in-affecting-understated-50-50
It's not often that I think a "here's my experience of something horrible but ultimately life-affirming" movie has the potential to really break out commercially, but "50/50" is the sort of film that audiences love to experience together. It's powerful, and there's a real release to sharing this, which is one of the points of making the movie in the first place.