Heh, we just did. We're about to enter P2 as well.Still in the Pyramid of Shadows here (took a 3 month diversion over in Forgotten Realms). I think lots of groups started with H1 on release and to be fair it seems like people tend to level every month. I'd say a bunch of groups are coming to the end of Trollhaunt about... now.
We dropped a character right at the end of H2, largely for the convenience of our players. We knew it had to happen, and rather than just have the character poof away into nothingness or get randomly killed, we plotted in advance that when the party got caught in the pyramid from H3, he would get stuck outside. Additionally, Splug got stuck inside the pyramid with the PCs (whole thing was Splug's fault, actually). He's a semi regular NPC now, having "won" infernal pact warlock powers in a bet that seems to have been less and less of a good idea as time goes by.
When we got out of the pyramid, we were dropped into P1, and had to solve it to get back to our original starting point, in pursuit of our long term goal of reuniting with our lost party member. When we did, we found that he had set himself up as the sheriff of the seven pillared hall, and after a short scene where we permanently displaced Brugg (and got him a new job as a city guard since one character is a softy), he gave us information that's acted as our hook into P2.