I got this the other day, and then completed it later that same day. £17 for 5 chapters was a bit steep, but I think worth it. Each expansion pack is like a story:
HL2 - The fight to destroy the Combine
Ep1 - The escape from City 17
I expect they'll carry on like this, seperate, linked stories for each expansion pack. There's meant to be 4, 3 by valve and 1 by another company. I think the next one will be taking the fight to the Combine, or saving Mossman. Then have the 3rd one in Xen maybe, trying to close the portal, with the final part cleaning up against the combine and maybe taking down the G-man and his "employers".