I've been curious about this title; it has 5 games, is available for $20, features critically acclaimed games, etc.
 
BUT, I have a question for all GBers out there; is it still worth buying?

FUCK YES. 
For Portal alone.

It's one of the greatest deals I've seen, back when it was full price. 
 
So yes, for 20 dollars, it's amazing.

Is Half-Life 2 and its episodes still one of the best games of all time?  When you can answer that question, you will know your answer. 
 
HINT: Yes.

Thanks; I might check it out.

Yes.

Why not

THE CAKE IS LIE. that means yes.

Absolutely yes. I didn't touch much of the Team Fortress stuff but for $20 you have all the Half-Life stuff there and Portal to throw into the mix.

depends on if you've played half life 2

Get it just for the fact it was the greatest gaming deal ever conceived.

Ok, but here's another question; do people still play Team Fortress 2 online?

Yes. Even if the price hadn't dropped a cent, it'd still be more than worth buying.

Yes and yes. Buy it NOW!
         
                ...NOW!
                                                       ... Now!
       ... Now
                                                                 ...now...

Yes, people are still playing TF2 online. Most people play the player matches and not the ranked.

" Ok, but here's another question; do people still play Team Fortress 2 online? "

Yes. Preferably on The PC. due to the much much bigger TF2 online community   

Yes, yes, yes.
 
HL2: Still worth playing, especially if you haven't yet. 
HL2 - Ep1: Arguably the weakest part of the HL main games. Still good, though. The developer commentary rocks! 
HL2 - Ep2: Amazing.  (Commentary!)
Portal: Must have and by far the best game of this generation (Commentary!)
Team Fortress 2: Fun, but rather pointless on the console version due to the bad performance and lack of upgrades + community. I still had plenty of hours of fun with it on the 360. 
 
 
Everyone should have The Orange Box.

On PC? Dude, yes. 
On console if you must? Dude, yes. 
 
The Orange Box is a must have for anyone, speaking of which I should get back to my billionth HL2 playthrough that I've started. Also, as Meowayne as stated above me -- the commentary in the games is awesome.

" Ok, but here's another question; do people still play Team Fortress 2 online? "

They're still playing on 360, however.

" They're still playing on 360, however. "