It's been about 4 months since the Guardian Cub was released in the official Blizzard PetStore. This pet was the first of its kind, being a single-use, tradable vanity pet (after a 24 hour CD) that's purchasable for either RL currency or in-game gold.
So was the implementation of this type of companion a hit or a miss? Well it's hard to say since I don't have any hard numbers or access to Blizzard's data, but over the span of the past 4 months I've noticed a large decline in not only the number of Guardian Cubs up for sale on the AH but also a decline in price. This leads me to believe that players are buying the cub (with RL money) and keeping it for themselves and/or most don't see much point in investing in them.
I don't know what the RL money to gold conversion rate is these days, but back when this pet was first released $10 (the price of the Guardian Cub) could buy you about 7k gold. The pets were going for about 10k originally, but now I rarely see the cub being sold for more than 5k on the AH.
At the moment I'm thinking the Guardian Cub was a neat idea but a miss overall. It's a good way to help empower individual players by giving them the choice between paying RL money vs. in-game currency as well as "buying gold legally", but in the end I don't think it was worth the resources to implement. And I'm sure it took more time to implement (compared to the other PetStore pets) due to its unique CD, single-use, and tradable status. Not to mention it's super adorable and has a one-of-a-kind model. :P
We'll see though. If Blizzard sees the release and sale of this type of pet a success I'm sure we'll see more vanity pets with a similar CD, tradable, single-use features in the future.