The Fanatic continued a trend of aggressive strong creatures being introduced into Magic. When he was first spoiled, I was very much on the Xenagos bandwagon! In the same set was the very popular God, Xenagos, God of Revels, who has since become a mainstay in Commander and has appeared in other formats, too. But while “Xenagod” had staying power, Fanatic of Xenagos really only became a Standard role player.
While Fanatic of Xenagos was a good creature in Standard, he didn't dominate the format in the way that many players first expected. He primarily saw play in one deck, Gruul Monsters, which was a very good deck. But he was more of a role player alongside Standard mainstays such as Boon Satyr, Goblin Rabblemaster, Heir of the Wilds, Polukranos, World Eater, and Stormbreath Dragon. He'd surface in the occasion Naya or Jund Aggro deck, too.
The good news for Fanatic of Xenagos was that he saw play for pretty much his entire life in Standard. He was still being played as late as April 2015 in Gruul Dragons, which is basically Gruul Monsters, but with Rattleclaw Mystic and Thunderbreak Regent. But since, he's faded a bit into competitive obscurity.
Still, though, Fanatic of Xenagos does still see some play today at the kitchen table. He's never been that valuable card, though. Fanatic of Xenagos' price peaked at $1 at release, and even the Friday Night Magic promo version never broke $2. Being an uncommon and not a rare certainly hurt his price ceiling, though.
If you're looking to build a casual Red/Green Aggro deck to play with your friends, Fanatic of Xenagos is a nice card to consider. He may not be the best three-drop that you'll find, but with such a low price tag, he's a fun card to play with.