My opinion, is if you like the story line and HL2 alot get it. Lacks any real new content though. Infact I can count it on one hand.
-2 new NPC (One of which was meant to be in HL2 anyways.)
-HLDM: Source (Which I am loading right now)
Level are of excellent quality though, but in typical HL fashion there is more questoins brought up, than answered.