I'm not at all saying they're bad games. I enjoyed, at the very least, about 6 or 7 hours of gameplay from each installment. The games are just too drawn out. It is possible to have too much of a good thing; they inevitably ended up feeling stale and just plain boring. It's like riding on an awesome roller-coaster with tons of exciting twists and turns, and it feels like nothing you've experienced before- and then 15 hours later, you're still on the roller-coaster, begging God to kill you with a random bolt of lightning because you've already vomited out all of your entrails.
OK, maybe that's a tad dramatic. To sum it up, I'd still say the Half Life series is pretty fun but suffers problems with the pacing and length of the games. When you have such a long game, it's hard for it to not get repetitive, but I felt myself slipping into these cycles of "OK, new area, not much action" to "oh I have to do something, slightly more action" to "TESTOSTERONE IS POUNDING IN MY EARS LEVEL OF ACTION", and then back to the first one. There's great planning and juxtaposition and everything you theoretically need to keep the gamer hooked, it's just too fudge long. I actually remember hearing somewhere that just under half of everyone who played Half Life 2 actually finished, but I can't find that link anywhere now, and I don't know how valid that is. Still, it wouldn't surprise me if that was even remotely true.
And hey, I'm not a guy to complain about length, typically. My first playthrough of MGS4 was around 12 hours and 30 minutes. That's watching every cinematic and listening to every Codec conversation. As you can imagine, that alone took up somewhere in the vicinity of 6 hours. Yet I was hooked for about three days straight as I feverishly tried to finish the game. On Oblivion, I spent just over 100 hours on my primary character, often playing for several hours a day in a period that spanned roughly two months. I loved Final Fantasy X, even though it took me goddamn 60 some hours on my first playthrough, and I bypassed most of the optional quests.
So it's not the length itself which is bad, it's the fact that the games within the Half Life series follow a design that would work better if they were shorter. Long games can be great, but to me, the Half Life series has always felt more stretched out than just 'long'.