It is a good film. Often referred to as the best Spanish horror film of that era. An odd zombie film- they are of course blind, so they track their prey by sound. They are intelligent, though slow. They also have zombie horses!
The plot is fairly standard- the remains of an evil castle, tourist gets separated, people go looking... The ruins are just creepy. The film has a nice atmosphere to it.
I own the coffin set but I have only watched the first film so far. Azz is right it is the Atmosphere that makes those movies special. The Zombies are creepy because they can hear your heartbeat so you can not hide from them. I loved the horseback scenes.
Films #1 & #2 are good. Haven't seen 3 or 4, so I've read some reviews. This is a good one that covers all 4 films, but it has tons of spoilers. To summarize it, #3 is kind of weak, but #4 is almost as good as #1.
At $45 for the box set, that sounds like a fair deal. With shipping, you're talking about $12/film. Hope this helps.
Tombs of the Blind Dead and the 4th, Night of the Seagulls, are the two best. The second is good too. You could skip number 3, but it's not so bad either, just kind of pointless. All are worth watching. If you see the boxset for cheap, get it.
Edit: For $45 it's well worth it. I think I paid 70-80 from Blue Underground a few years ago.