I've been playing for about 6 hours now but I take my time in FPS, especially HL... Try to get in every nook and cranny and generally just run around pushing things over, breaking stuff etc.
So 6 hours and I am on the second to last chapter... I hate that it's $20 but am I enjoying myself... yes. Of course everyone wants everything to be cheaper but I don't think $20 is all that bad. You can tell the developers put a lot of work into this. It's not just rehash. It feels better and looks waaay better than HL2. I'm actually having more fun playing this than I think I did HL2 [after the novelty wore off that is].
I also have the game set on hard and I die a lot but I like to be challenged in an engine I've essentially been playing for 6 years [counter-strike and countless HL/2 mods].