Other than the Celestial Steed, I also bought the Wind Rider Cub plushy and in-game pet set. And the Spousal Unit has the Winged Guardian mount. (I think he's eyeing the Armored Bloodwing too. Hmmm. Father's Day is coming up ...) In addition, we each have the collector's editions for several years and all the pets included there.
As much as I love my purchased in-game goodies, I do have a bit of buyer's remorse about spending money over and above my subscription. These digital goodies are really only mine to rent while playing the game. Should I ever stop playing WoW, all that money will have gone to things I don't really own. Still, I've been hanging out in Azeroth since the open beta and am still loving the game, so my departure from World of Warcraft isn't likely to happen anytime soon.
Have you bought any in-game items from the Blizzard store? Do you regret the purchase after you see how popular your new mount or pet is? What are your thoughts on spending money on virtual items?