I, Borg is a great follow-up to the first two Borg eps. After the wham-bam action of BoBW the only thing the next Borg show could do was go totally opposite, make it small and personal, and that's exactly what they did. It's a fine episode.
Descent suffers from having one vaguely good idea - what if Data feels rage? - and not having a clue what to do with it. Part 2 is the weakest conclusion to any of TNG's two-parters, it does not answer a single question Part One asks, and is the clearest illustration of what was always said, that Part Ones were written without any real idea how to finish it. It's a mess, full of bizarre plot points (all the crew on the planet - what?) and a huge drop in quality compared to the other TNG Borg shows.