Well dunno if the Iberians count as Celts, but I guess this might be a good place to talk about it. The Iberians seem to like their Iberian Velites quite a lot, because I've been killing stack after stack of Iberian Velites as the Karthies.

Of course they sometimes have other units like Caetreatii Loricatii, Scutarii and Curisii, but they tend to consist of about 10-15% of the Spanish armies. But this sort of seems to mostly be an early game trend, because as the game has been progressing and me constantly killing off about 10 units of them yearly, I've noticed that the amount of better units has increased.

I'm currently playing a normal/normal game as the Karthies and I'm in 251 BC.