Do you mean Blue Shift and Opposing Force? They're pretty decent in my opinion, especially Opposing Force. Given the choice though I'd go with Half-Life 2.
No Mutants Allowed
Prone Squanderer
R.Graves
I've played half-life 2 and know its greatness but I just learned about opposing force and blue shift... are those new experiences worth more than half-life 2 though?
Prone Squanderer
They don't add that much but they do explore a bit more of the Black Mesa complex and offer some new gameplay (well Opposing Force does).