The ending, as many reviews have said, is a bit of a let-down. You don't get much of an answer about how the aliens got to Earth, what they want with humankind, and why Dr. Breen is so convinced that they will make humans into something better than they can ever imagine. But you do get the opportunity to blow up lots of stuff, so I guess that's something... ahem...

Now that I've beaten it, I've taken the time to figure out how the cheat codes work and put them to good use early in the game. I've gotten to see and do a couple of things that are impossible early in the game, like having a shotgun from the beginning of the game and blowing away the cops who knock you out initially. Of course if you do that, you won't hook up with Alyx and the game comes to a crashing halt. But you do get the opportunity to see some things you wouldn't have seen earlier (like the rooms the cops come from and the view outside them).I'll probably use them to do some other exploration later.