The genius gag behind "Grudge Match" is pairing Sylvester Stallone and Robert De Niro as two rival boxing champs with a complicated history; it's "Rocky" versus "Raging Bull" by way of an anti-buddy movie formula. The genius fizzles there, and the adequate takes over, but that doesn't mean there's no fun to be had.
Stallone plays Henry "Razor" Sharp, a working class guy who was swindled out of most of his money by fight promoter Dante Slate (Anthony Bean). De Niro plays Billy "The Kid" McDonnen, who spun his fight winnings into a chain of car dealerships and a nightclub (where he indulges himself in delusions of stand-up comedy). Their financial fortunes are worlds apart, and so are their states of mind; Razor can't scrape up enough cash to keep his elderly trainer, a curmudgeon named Lightning (Alan Arkin), in his retirement home, but he's content to let the past remain the past. The Kid, by contrast, wants nothing so much as to get into the ring with Razor one last time, to settle once and for all the question of which man is the better fighter. Their two bouts resulted in one victory for each; the third, highly touted "grudge match" that was supposed to have taken place never did, due to Razor suddenly retiring from the sport.
When Dante Slate, Jr. (Kevin Hart) approaches the now-older fighters with a proposal for some quick cash making a boxing video game, Razor only reluctantly agrees. The Kid is so keen on the idea that he barges into Razor's motion-capture session and sparks a fracas that, recorded on cell phones, becomes an instant viral sensation. Smelling money, Dante Jr. puts together a deal that will pay big bucks to get the two into the ring for real. But things are complicated -- there's an ex-girlfriend in the mix, a stylish blonde named Sally (Kim Basinger). There's also a long-lost son (Jon Bernthal, landing on his feet in the wake of his "The Walking Dead" character being killed off). Old wounds are opened, new bridges built, and there are episodes of dumb, but funny slapstick as well as moments of family bonding over crude sexual humor (the result, in one slightly belabored scene, of Bernthal's character being nicknamed "BJ").
Writers Tim Kelleher and Rodney Rothman have some fun, using archival images and visual effects work to create a record of rivalry between the two champions and gleefully riffing on the "Rocky" films. Burgess Meredith not being available for training and mentoring duties, Arkin fills the role quite nicely; Bernthal provides the hunk factor; Basinger keeps her turn as Sally understated (the best choice, given the thinly written role). If nothing else, it's interesting to see how the cast's four Oscar winners bounce off each other, but Kevin Hart brings a mouthy, cocky revelry to the film that gives it a little extra juice.
In short, this is a decent enough movie: Entertaining, sprinkled with some good one-liners and enough physical comedy to keep things moving, and with a workmanlike, but more than adequate, story to keep things on track. There are probably better ways to spend Christmas Day... but if not, you could do worse.