I love, love, love the trilogy. Though nothing is flawless, these books are some of the best in the series.
This probably as high as the stakes get before the final arc: Keeping the blue box out of enemy hands, finally enlisting other humans to the fight, and having to keep a number of world leaders safe; no where else do so many things of such importance have to be balanced. (Let's face it, even before the filler stretch, a lot of the books where spy out on person/new tech and escape alive).
This, and the novel threat that was David, that there was someone using there powers against them, so damn effectively, is spectacular, and brings out a lot of the best, and worst, in all the characters.
But for the bad: I don't really mind David's lack of development. He's 14, course he's crazy, but I like that it isn't some drawn out descent into madness. That really he's always kind of sociopathic, plotting his own ends and quickly excepting that in his new world he can pull different strings in new ways. He didn't develop because, that's always who he was, he such had new tools to work with to let it out, I think that's brilliant.
What did bother me is how quickly he switched focus, because it was a function of the series structure, not his character. In 20 he and Marco but heads, in 21 that tension's largely forgotten so he and Jake can build up to a head, and then finally moving it all to Rachel in 22. It works the way it was written, but it was written that way because it'd be weird for a book told by Rachel to center on the conflict between David and Marco. It would have been nice if those match ups could have been resolved, or left to one.
The other problem was the world leader issue. It's classic Animorphs nothing gained, nothing lost, but that kinda sucked when everything else was so grandiose. We know one of them is a controller, but that's of no consequence ever again, no allies are made, V3 isn't even properly punished for failing on that scale. That plus some sloppiness of 5 vs 6 at the meeting, that element was always something of a let down.
But on the whole, great set, laughs, tears, gasps, everything that makes the series great is on full display in the trilogy. Love it love it do