I still didn't see people using them. So I don't know how relevant this unit/diversification really is. Maybe people didn't realize they can make them in barracks. :/ But I really like the idea of this small differences, like the Iberian fire CAV.
We should make champions available as soon as one enters P3. There should be no extra tech for it. Also there should be a balance for CIVs, which need extra buildings for champs. Some of that buildings have high costs or bad unit stats (the Athenian gymnasium still doesn't extend the area of a player).
My experience with champ build time in team games is not that bad, when I start fighting there wouldn't be enough resources to make constant champions with reduced building time. So I don't consider it one of the biggest issues in 0AD.
Rebalancing archers would be more relevant, people again complain about getting archers CIVs, when games are with random CIVs!