Genndy Tartavosky is notable for shows like Dexter's Laboratory, Samurai Jack and Star Wars: The Clone Wars, so it's no surprise that this show is great as well.
This show has beautiful visuals, a proper and subtle mix of 2D and 3D animation, diverse character designs, and an excellent soundtrack to boot. The cast consists of voice acting greats like Tara Strong, John Dimaggio and Keith Michael Richardson, who all do a great job in portraying their characters in believable light.
The plot is simple. An alien king sends his daughter, a royal knight and a robot to earth to save them from a traitor's wrath. The trio land on earth and must assimilate with the humans and avoid be found out by the military. They can transform into robotic warriors and can combine their powers to create a sort of mega-warrior to defeat larger Godzilla-like foes.
Oh yes, When I said there was mature themes, I don't mean things like excessive sex, violence, swearing or drug/alcohol content. Well, mostly. There is a lot of overt violence during the monster fights and implied violence in the destruction of cities both on Earth and the alien planet, Gallaluna. The hand to hand combat throughout the show is comparable to that of the Mech on Monster (called Mutratti) combat. The sexual themes die out early, but there's a notable scene in episode 10, but other than that the show's fairly tame. There are no bad words or drug content, and the show is fairly positive otherwise.
There are definitely a lot of great messages of family, unity, determination, and themes of loss, perseverance and being different. Illana represents the Heart, Lance the Brawn and Octus is the Brains. The humor isn't inappropriate and there are a few references that might go over kid's heads.
Sym-bionic Titan is a great show and you might be able to find it online somewhere.