I started reading this series in 2001. It's been 16 years and there's still no end in sight.
Don't get me wrong, I think Case Closed is great, with one of the most solid, intriguing main plots out there in the detective genre. I don't think anyone can deny that Conan is a superbly smart series with lots of well-thought plot twists and characters.
However, by around volume 50 or so, the random cases (so-called "fillers") really got on my nerves. They dilute the main plot, and some fillers are just ridiculous. You can use simple common sense to figure that certain tricks and gimmicks are downright unperformable. Sad to say, but it is quite clear that Aoyama-sensei must be running out of ideas! And I'm really tired of the continual introductions of unknown, mysterious characters. Yes, I understand that they play a huge role in leading Conan to the Black Organisation, but come on - after like a dozen of them, isn't it time to unfold the Big Boss?
Lately, I don't even bother to read the cases. I just picked up the chapters that have something to do with the Black Organisation. Well, what can I say? I'm one of those fans that have grown frustrated.
All in all, I'd like to say: Read this series if you haven't. After all, it is like the biggest detective manga out there; and as I said above, the main plot surrounding the Black Organisation, the CIA, the FBI, the immortality pills, etc is fantastic. But if you are looking for something that is really focused on mystery and murder tricks, Kindaichi is probably a more fitting choice.