They're alright. I haven't played the ones before HL2, and personally I think HL2 and what I've played of Ep1 is... mediocre.
I just can't get into them. There is so much hype about them that when I bought the Orange Box off Steam, mainly for TF2 and Portal, I thought that I might as well play what was supposedly every PC gamers wet dream.
I admit I was hooked at first, but the interest slowly drained away, and when I finish HL2 I realised I wasn't looking forward to playing Ep1. This generally isn't a good sign in games, where you have bought the games sequel, but have you force yourself to play it simply to try and rationalise the money spent on it.
By no means are the Half Life series bad games, but I don't see why they are the most amazing thing since sliced bread.
Just my honest opinion. I've tried to explain it the best I can. I did not enjoy the Half Life series.