Written by Toshiki Inoue back before people would shake their fists and cry out his name should a character fall into a pool of water at the end of a fight, Kamen Rider Agito was an exciting watch for me, one that always left me wanting more. Although similar to Kamen Rider Kuuga in many ways, the story unfolded very differently and that's good because simply making a straight Kuuga rehash would dull the quality of the original.
That said though, Agito does fail in a few, specific areas which undermined the total enjoyment factor for me. One of the first and earliest grievances I had for the show was the mishandling of Agito's various forms, which were all functionally the same on screen with the exception of which weapon Shouichi had at his disposal. Whereas in Kuuga, Double, or OOO the various strengths and weaknesses of each form were evident, this is not the case in Agito. For instance, the Storm form is supposed to be the equivalent of the Dragon form from Kuuga, but we never see it function as an agility form.
Shouichi himself is also probably the least engaging of the three main Riders, simply a happy-go-lucky guy who laughed and smiled and wasn't bothered too much by his amnesia. Hikawa and Ryo were the ones that always pulled my attention in with their own struggles and drama. My favorite episodes and plots always had them at the forefront and I could get more in touch with them. The characters they interacted with, especially the Akatsuki passengers and the various police officers. It probably hurts Shouichi's case that there is no real progress in finding out his true identity until halfway through the story.
The Lords/Unknown were a case of hit-and-miss. On one hand, I appreciate the plot arcs featuring more than one monster of the week at a time. With three Riders, upping the number of enemies they fought on a weekly basis gave them all time to shine. But at the same time though, the Unknown lacked personality. They'd show up, kill people, and be done with it. They're obviously capable of speech, so why couldn't we ever see them interact on their own? One of my favorite parts of Kamen Rider Kuuga was just that: when the Grongi would talk in their own language. It reinforced their foreignness while at the same time gave them personality.
I do recommend Agito; it's a good show. Just don't expect perfection.