Firstly the mysterious Chapter 15 is the credits is not a bonus level or anything secret or special. Sorry to burst your bubble, but I think that slot is misleading.
Although I do recommend watching the credits all the way through.
I don't know if I even want to post an opinion at the moment because you aren't going to like what I have to say.
I felt that the whole game, from plot delivery to the street fighting, feels just like Call Of Duty.
I know one is Sci-fi and the other is World War 2, but they just feel the same. From people talking to you without expecting a response, to the way you climber through broken buildings and have teammates constantly standing in your way.
Both games have infinite streams of enemies that have to be mowed down by inaccurate rifles and some of the levels even look similar (COD's last Russian level is similar to the end of HL2's chapter 12). I preferred HL2's sniper missions to those in COD, but then I preferred the vehicle missions in COD because they never got boring (HL2's drags out).
On both you get pushed around by the enemy when you get too close to them, on both you are always having ammo problems with the rocket launcher and on both the rifles don't seem believable because they are so painfully inaccurate.
I am not saying they are the same, but they both feel like part of the same franchise. HL2 does not feel like the sequel to the original Half Life because it's pacing is a lot faster and there is just so much street combat, which wasn't even in the original.
I was kind of hoping the plot would be both expanded and some of my long standing questions would be answered, but the game simply doesn't reward you with plot points. Sure, there are scripted sequences as rewards, but not a single one will answer any questions you might have or specifically expand on the universe by providing new information. It's just glimpses here and there, or partial overheard conversations.
Some bits are cool, like Farther Gregory, the first half an hour of each vehicle level, the final level because it's back to that good-old slow paced exploration gameplay of the original, and the comedy elements are good, but I do wonder if Valve are catering too much for new players because there was not a single point in the game where I was glad that I had played the original or found a joke that only someone who had played the original would find funny.
A lot of the enemies from the first one have been removed and actually HL2 has far less variety in both enemies and weaponry. It's a good game and it will get rave reviews. Probably because nobody will dare give it a poor score through fear of a fatboy backlash. I know Valve have put their all into this, but it doesn't feel like a worthy sequel, simply because the heritage left by Half Life, Opposing Forces and Blue Shift isn't tapped into at all.