Gekiranger is a very enjoyable season for a number of reasons but perhaps one of the most enjoyable aspects of the season is seeing the two primary villains, Rio and Mele, grow and develop into characters.
The usual stand point of upgrading villains in a Super Sentai is normally after a series of defeats they either gain a new member who provides a new challenge, they rapidly upgrade in a single episode, or they gain a new weapon or ability that the heroes have to counter.
In Gekiranger, Rio and Mele go through a progressive upgrade. Rio doesn't just sit around and observe while the Monsters-of-the-Week get destroyed. He trains and gets stronger, preparing for the day he fights his opponents, as well as awakening the Kenma to train him, even nearly getting killed in the process of his training.
Mele was a huge surprise as well. She appears at the beginning as a love-sick girl who annoys Rio. But Mele herself also begins progressively getting stronger, though her motivations were more to help Rio than for herself.
These two progress in getting stronger at the same rate that the Gekirangers do.
As for the rest of the show, there are the average pros and cons. The Mecha doesn't seem all that special, though still fun to watch and the starting weapons seem bland. I do like the various teachers the Gekirangers have as well as the training, but after training there wasn't much for them to do.
The Gekirangers themselves don't stand out much, and don't seem to have characteristics that haven't been seen in Super Sentai. The suits were okay, but the core three lacked uniqueness from one another, and I didn't care much for the mid season power up. The two additional rangers are OK in their individual episodes but overall, not really too notable.
Overall, I'd give the season a 7.3/10