I still haven't finished HL2 or even touched the two add-on expansions
The two Episodes weren't that great. But you should really finish HL2. The mad rush of storming the citadel is awesome!
I thought Episode 2 was great, but disliked Episode 1. Episode 1 found me getting incredibly sick of the zombies moaning, then followed it up with babysitting NPCs. I didn't like it. If I recall ammo was an issue, too, making the zombies more time consuming.