Mega Man Star Force plays it safe and doesn't toy with the Battle Network formula much, but it's still a great game.
The general gist of Mega Man Star Force is that you are a young boy named Geo Stelar (or whatever you wish to name him - you're not stuck with the given name for a change). Geo is the son of an employee at NAZA, an space exploration institution much like our NASA. After his father becomes presumed dead following an unknown incident aboard a space station, Geo becomes immensely reclusive and obsessive about space, hoping to one day hear something from his dad, unable to accept the idea his dad is dead.
Following this event, Geo doesn't attend school, and he has no friends until a chance encounter while stargazing introduces him with an alien being named Omega-Xis, or Mega for short. Mega is a creature composed of electromagnetic (EM) waves from the planet FM, visible to Geo thanks to his father's visualizer glasses. He learns shortly thereafter that he can fuse with Mega and, in doing so, transform his own body into EM waves, enabling him to walk on EM paths wherever they are located, as well as enabling him to transmit himself into electronic devices.
If this sounds familiar to Mega Man Battle Network, that's because it is. The viruses that were your main enemy are back, although your main boss enemies are no longer Navis, but instead FM-ians like Mega. The transmission of an entity into electronics is also back, although in this case your character is transmitting himself into them instead of his Navi. The method by which this is done has also been changed: you now must first transform into your EM wave self through "wormholes", found scattered throughout the land, and then make your way to the electronics on the EM paths.
If there's one main reason I could give for why I like this game, it's because the game is just good clean fun. Gore and violence is all well and good, but no matter how old I get, I'll always hold a soft spot in my heart for a story, script, and art style that one might expect from one of the higher-brow Saturday morning cartoons. There's something that is simply fulfilling and satisfying for me about playing a game where nobody dies and where everything turns out right in the end. If you disagree and would prefer all your games to be dark and gritty, Mega Man Star Force definitely isn't for you. But for me, Mega Man Star Force's enduring innocence and playfulness is something I can't help but find endearing.
That isn't to say that there isn't anything here for people over the age of twelve, however. As might be inferred from the above, the story is not exactly the game's strong point, but it doesn't make you want to go kill yourself, either. And its diabetes-inducing happy moments and morals conveyed can't help but put a smile on your face, even if they do simultaneously seem rather corny and unrealistic.
The battle mechanics also play well, although, again, it's fairly similar to that of Mega Man Battle Network. The major changes are four points: the 2D sprites have been replaced with 3D cel-shaded models, the camera is now behind Geo instead of in the middle, you can't move outside of the back three panels anymore, and, to make up for the last point, you can now perform a lock-on to enemies which will result in you running up to them and smoking them in the face with whatever weapon is currently up for use.
While the rest of the battle is identical to Mega Man Battle Network, the changes feel just fundamental enough to make it seem like a new experience, and the last two points work to solve one of the biggest gripes people had in Mega Man Battle Network, which was that weapons were entirely useless if an enemy wouldn't come within range.
So here's the bottom line: should you get Mega Man Star Force? If you want all of your games to have violence and deep stories with twists and turns, then no, Mega Man Star Force is not for you. If you have a very distinct inner child who just won't go away, however, then Mega Man Star Force has plenty to offer you.