You. Must. Own. This.
If you've played the original (part of the Half Life 2 Orange Box) you'll have a good idea what you're getting into. I found that there were were fewer moments in this one where you felt lost at where to go next. The puzzles, while incideous, are padded with just enough visual cues to guide your thought process in the right direction. The rest is up to you.
And most importantly, yes the graphics are a significant improvement on the already suitable original, and yes this game is freaking HILARIOUS! You'll come for the unique experience, you'll stay for the characters.
I couldn't recommend this highly enough. There simply isn't anything else like it.