That they're completely unnecessary until you get to ROTS. That movie is like an anthology of bullet points that should have been hit in all 3 movies, and it suffers for it.
But that's why many people liked it better than the others - it gave them what they were expecting out of the first 2 movies.
I still maintain that TPM is the best of the prequels, since, while it doesn't matter in the grand scheme of Star Wars, it has the most cohesive story of all the prequels.