I've been out of the UK during the rise of Greggs, so I've only had it a couple of times. And both times, it's been a pretty small, lukewarm pie (well pasty, I guess). I had a similar experience with Nandos, which I heard people banging on about for years before I had one. And then the day finally came and I realised that it was just pretty dry chicken without much flavour. It's really popular here in Malaysia, which is insane to me, because Malaysian chicken rice shits all over it and is half the price.
We could add Subway to the list, incidentally. Shit bread that would embarrass any bakery on the high street, yet inexplicably the most successful fast food chain in the world. And I've contributed to that success a lot, because they used to have a branch next to my work. And they're fast and convenient, which when you're a lazy, disorganized prick like me (and most people) who can't plan ahead to bring food to work, you just go for the place that gets your full ASAP. They did once look at me like I was a mental patient because I asked if they had butter, as if that wasn't a completely reasonable request for a sandwich.