Yes, they started to slip into one-dimensional characters near the end. Not completely, but slightly. That's one of the reasons that I dislike Rachel in the later seasons, and why Joey's humour went downhill.
But the biggest change was that of Chandler. Now I loved his relationship with Monica, they worked really well together, especially in Seasons 5 and 6. But then they got engaged and Chandler slipped into a rather dull, mature Chandler. Ross used to be the mature one, I miss the Chandler that would make chocolate milk and play Hide and Seek with the Chick and Duck!