Some folks will say very. Some will say not at all. And many books will sell without them, and you’ll probably find advice all over the internet and in marketing books that says get reviews! (here’s some interesting info about common Amazon myths featuring reviews).
And while reviews are in no way the be all and end all, in the end I agree to an extent that they’re important for two reasons:
1. Some readers do like to see that a book has a few reviews and some will read those reviews. Not all readers, but some. Having some reviews on your book will cover that group.
2. Many promotional sites (more on these in a later post) require that the book you list with them has a certain amount of reviews before they list it.