Let's start with the synopsis from wikipedia:
A group of young pirates come from space to Earth to obtain the "Greatest Treasure in the Universe", which can only be acquired after obtaining the "Greater Powers" of the different 34 Super Sentai Teams. However, they end up facing the Space Empire Zangyack, whose earlier invasion forces were wiped out by the 34 Sentai groups long ago before a new invasion force is established under Commander Warz Gill, the emperor's son. As a result, due to their history with the empire, the space pirates use the powers of the older teams and attempt to master their powers to fight the Zangyack forces as the Gokaigers.
Warning: Gokaiger is full of spoilers of older Sentai series, so if you don't like spoilers and intend to watch some of the previous series, you should be careful.
Gokaiger was a good series. As you can tell by the synopsis, it's full of fan service. And they even brought back the actors that played the older Sentai members to do cameos during the series. The only thing I didn't like was that 5 of the Sentai didn't have their own episodes. The show had plot reasons for that, but this doesn't make it less disappointing.
The figths were usually very good. The choreography was cool and they used the fact that the Gokaigers can transform into any member of any previous Sentai very well, making even the less inspired fights interesting by using the techniques of the older Sentai. The series also had a lot of untransformed fights, which is always nice. Their big entrance shooting the enemies while being just in time to foil the enemies' plan.
The giant fights were ok. GokaiOh was too bulky, so they couldn't do much in terms of choreography. In fact, I don't think it was necessary for each Pirate to have his own vehicle, since all of them are stored in inside Red's Gokai Galleon as if they were those russian dolls. They could've made Galeon, their spaceship, a single vehicle that became the mecha. The only good point in the giant fights were the giant henchmen. They added more possibilities during the giant fights and could be quickly disposed of in the beginning of the episodes that didn't have a giant fight against the MOTW.
Gokaiger had some good villains. Walz Gil, the heir to the empire was a very funny enemy. He was a spoiled and hysterical prince. The reactions of the commanders to his antics were also funny. Unfortunately, his father shows up and everything gets very serious and the remaining commanders have less and less space and end up being wasted. There is also a cool rival for Red.
The plot is pretty much what you can imagine by reading the synopsis. But it works fine, which is great, since the series is a huge fan service and having a sound plot isn't common in series like that.
The characters are also good, even though they aren't very original. But they are full of mannerisms and, what's most important, the writers made them interact a lot, including in the background of the scenes. This was not only funny, but made it look like a real group.
Red, Captain Marvelous (yes, that1s his name), is the bold red leader. He show off too much, but that's because he's a pirate. And as a pirate, he plays dirty, even if, at the end of the day, he has a good heart and helps people. Joe Gibken is the first mate and swordsman of the crew. Like most blues, he is the usually cool guy that, sometimes, show a very intense side of his personality. Luka Millfy, GokaiYellow, is a tomboy that is always after money and constantly hits green and the sixth when they are goofing around. If you think it's looking just like One Piece, you won't be surprised when I tell you that Gokai Green, Don Dogoier aka Professor, is a coward invento that uses his brain and an unorthodox fighting style to beat the enemies. Pink, Ahim de Famille, is an exiled princess and the one that keeps the group together by being nice and playing the mediator. Finally, we have the sixth. He's a Super Sentai fanboy from Earth. And he always acts like a fanboy when he sees one of the veteran sentai members. He's also very eager, unlike his mates that always act like they don't care.
Gokaiger has great fan service, great fights and great characters. And it manages to be exciting most of the time.
Veredict: must watch