I finally got the orange box a few months ago (money is tight so I've been waiting a while).... I was really impressed with the first Half Life... so I'd been really really eager to play the second.
When I new I was getting it, I played HL again so I was back in the zone... and even now, even with the not so up to date graphics, I still thought it was a really well done game. I do think you need to have played the first to really appreciate the second. If for nothing else it gives the storyline of why you've got the aliens running about and who the characters are (yes I suppose you could google it) - including yourself.
I've played through Episode 1, portal and lost coast... just about to start episode 2 tonight. When Team Fortress2 finishes downloading (I keep forgetting to resume it and let it run overnight), I'll have a look at that (I've played the first one a little)