Still my favourite WOW expansion. I will focus on the bonus contents, not the game.
The box itself is a sight to behold. Sturdy and artistic. The artbook has a high gloss feel to it, and features some amazing artwork. From watching the behind-the-scenes DVD, I found out that Blizzard really tried something different with this expansion. In my opinion, the most underrated expansion so far, with absolutely amazing locations and two fun new races. The soundtrack is still my absolute favourite WoW soundtrack, better even than the much acclaimed Wrath of the Lich King. It also comes with a nice Deathwing-themed mousepad. The in-game bonuses however are a letdown, as you only receive a pet, BUT an AMAZING pet. Lil' Deathwing is an absolute joy to have as your travelling compation and, as of now, as a battle pet. Unlike Pandaria and Draenor, it does not include an exclusive mount, but that started with pandaria so no big deal. I did not check the Diablo and Starcraft bonuses, but they seem cool.
Even now, when Cataclysm has become pretty much obsolete (you can however upgrade your standard Cata to collectors edition Cata by inserting the code), I suggest you buy this collector's edition if you're a big WoW fan. The goodies are worth the price, BELIEVE ME. It is high quality material.