It was a great, fun movie. The animation was amazing, the backgrounds beautiful. The huge cast of characters was slightly overwhelming at first, but by the end I loved everybody and it had me fondly reminiscing about my own family get-togethers. :)
Though, it also reminded me a lot of the 2nd Digimon movie, Bokura no War Game. It uses the same basic concept- our world today is so wired in technology that when some threat starts screwing with it, there's complete chaos. The heroes of the story have to fight the enemy on the internet to save the world, with everyone else in the world watching and helping out. In Summer Wars, the Digimon are just replaced with avatars and an A.I. program.
It turns out the similarity is no coincidence. Mamoru Hosoda, director of The Girl Who Leapt Through Time and Summer Wars, also directed the first two Digimon movies. After learning this, I decided to rewatch Bokura no War Game...
And now I can really see the similarity in style. There is a main difference though, and it's that while Bokura no War Game focused on children, Summer Wars put its focus on family. Basically, Bokura no War Game is a kid's film, and Summer Wars is a film about family for the whole family.
Personally I think that worked in Summer Wars' favor; while in Bokura no War Game, the adults seemed mostly unaware of what was going on, in Summer Wars we get to see the adults getting involved by pooling resouces from their various jobs, which was fun to see, and I especially liked how the characters drew on family history for inspiration in their strategies. It also showed more real-life consequences of technology going haywire, with casualties.
What I was most amused to see was how much technology has developed in the time between the two films. In Bokura no War Game, released in the year 2000, Taichi barely knows how to use a computer. When his dial-up connection gets cut off, Koushiro's solution is to use a satellite connection. They get tripped up in the fight by the Blue Screen of Death and the computer lagging from too much email. Yamato and Takeru, who are visiting their grandmother in the country, have to race around the countryside in search of a computer with internet access.
Compare that to Summer Wars, released this year, that also has the characters visiting their grandmother in the country, yet they are all able to access the internet through laptops, cell phones, and handheld games. There's a humorous scene in Bokura no War Game where Diaboromon prank-calls the world, and everyone in Japan is shown taking out their cell phones at the same time. Where the Sign of the Times in that movie is EVERYONE HAS CELL PHONES, in Summer Wars, it seems to be EVERYONE HAS INTERNET.
Anyway, both movies are worth checking out. Summer Wars stands alone better, but I'd also recommend Bokura no War Game even if you're not into Digimon.