i wish “mmm” was a reaction i could click on for this.

I've had one and they are, at least to me, pretty disgusting. The vanilla custard filling is totally bleargh worthy.

http://www.asylum.com/2009/12/30/searching-for-the-mysterious-mcdonalds-holiday-pie/

…but what IS it?

phlegmpocket.

They basically taste like old, batter-fried cream cheese hot pockets.  I remember I was at Disney World, and it was like midnight, and there was this woman in front of us in the drive-through. She was the only person in the car, and she was ordering basically every sugary drink on the menu before asking “what kind of pie do you have?” So the guy said “we have apple pie and holiday pie,” and the woman said “okay, I'll take two apple and two holiday pies.” Meanwhile, me and my entire family were laughing uncontrollably, because not only did that answer satisfy her (she didn't ask what was in the holiday pie), but she ordered two.

Hmm, hard to decide whether I'd want to try one or not.

I was told that they were discontinued last year when I was itching for my fix. They're not insanely delicious, but they are quite addictive.

They taste pretty good for things coming from McDonalds.
The McDonalds near my house have these and those McRib things. Yuck.