My priest has been disc since level 10 and leveled solely through dungeons and it's been a blast. At 40 I dual-specced her disc/shadow and used shadow for questing while waiting for dungeon queues, but her OS is now holy to try that side of healing.
I definitely like disc best, penance is by far my favorite priest spell (it's SO fun, too!) and I was ecstatic to finally get it. I really think it just depends on your own playstyle; I don't think holy or disc are better or worse than the other.. I just personally prefer disc. She's level 68 now and at 80 her MS will definitely be disc all the way.