Drat: I still haven't beaten HL2 or played the episodes. The first time I got to the bit where the Combine are blasting with some big turret thingy from atop a building (not long after you rescue Barney from the snipers). The second time I only got to the first bit in Nova Prospect where you set up the turrets.
In both cases my progress was halted by my rampant OCD forcing me to do bits over and over again because I was sure something glitched or otherwise didn't go right. Eventually my attention gets grabbed by something else and I stop playing the game for ages, and eventually decide I may as well start over. For the same reason, my attempts at replaying HL1 have generally been halted at that big diagonal lift with all the headcrabs raining down from above. But at least I've played through that several times and the expansions more than once.
My finances are approaching the point where I can get a decent new computer, so I might as well wait for that so I can play HL2 in its full glory. Kind of well timed considering they've added in the Xbox achievements.
The segments with the turrets are the hardest in the game. Nasty stuff.
I must have started over on FFX-2 at least four times by now. Each time I'll start playing, have a good time, and then I'll check a walkthrough for something and realise I missed out on two thirds of the content, promptly losing the will to go on. Each time I resolve that next time I'll keep a 100% checklist next to me as I play, and each time I make the same basic mistakes.
Now that I think about it, I think I've resolved to replay about half my backlog from the beginning since most of them have fairly intricate stories and I don't want to miss out on anything. The exception would be FFT, of course, since you can replay every cutscene from the in-game menu there. (But not the in-fight dialogue).
But, yeah, The Orange Box. It holds up amazingly well - it's barely aged at all. I know a lot of people who only played through the games in it quite recently.