Sure, they each have their main features that never really changed, but it never got stale. From the last season compared to the first, only a couple of the characters are really similar.
From the outside looking in, it may seem that there was little character growth overall, but that's not totally true. Each member of the group and many of the co-stars had very strong personalities that never got boring or played out. Friends has six of the most likable characters in television history.