On a related note, how much books are you ready to try to _get into_ a series?
Most of my friends were engrossed in George R. R. Martin's series at some time. I read the first book and didn't really like it, and they told me it got way better after the first book.
In a similar way, I've heard people say that the discworld series by Pratchett starts to be really good only from the 5th book or something (I read the first 3-4 books and found them enjoyable, but not enough to make me want and read the rest).
So, in the case of a highly praised series, if you don't find the first book really enjoyable, will you try and give the next book a chance?