Like goplanet said, I wouldn't recommend buying any game without trying it first, unless you have no way to do so. If you come in here (Blackwell section of GOG forums) asking whether the games are good, or not, you'll probably only read biased opinions praising the games, when it's actually and always a matter of perspective and what you think is good or bad and, ultimately, what you want from a game. I would say you shouldn't let others decide for you. Blackwell is a great series of games if you have a craving for old-school point & click adventure games, such as those made famous by Sierra, LucasArts and the early Revolution titles. They're not long games, but neither of them is as short as one might think, at first, given all that's been said. The story is amazing, if you're into that kind of thing, but it isn't particularly deep or extremely thought-provoking (check two other Wadjet Eye published titles, Resonance and the superb Primordia, for comparison). There's humor, and it's pretty decent, but don't expect the kind of quality humor like you find in Monkey Island, Broken Sword or Grim Fandango. Gameplay is pretty standard p&c, though adding notes for you to mix and immerse yourself in the investigations is a nice touch. The retro styled presentation is lovely and extremely polished, something all of us that have been keeping up with Wadjet Eye's games had come to expect -- the games look really great but, again, I'm not sure whether this retro approach is your cup of tea, or not (nothing wrong if it isn't, we all look for different things, and that's more than fine). The voice acting gets the job done, but it's nothing to write home about -- although I'd have to say Abe Goldfarb is amazing and clearly stands out. The music... oh, god, the music! To be honest, I found the soundtrack to be pretty mediocre, and was glad that I could turn the music off entirely by Blackwell Deception, something I immediately did; the exception being Unbound, since they went for a more "jazzy" approach, and it's the one game in the series whose soundtrack not only isn't bad, it's actually top notch.
As you can see, opinions do differ, so try the games for yourself first.