I don't think character names can make or break a book, but I do think they are important. If the names are not just right then it can create doubt as to the authenticity of the story. When I read Thirteen Days To Midnight by Patrick Carman, the character Ophelia was nicknamed "Oh". It just got confusing for me whenever Oh came up and it wasn't part of dialogue. Sure I got used to it, but in the beginning it was a bit jarring.
I stumbled across this article called Name That Character! that gives some great tips on the subject. And here's an interesting piece 16 Great Characters With Numbers for Names. And finally here's a list of 100 Best Characters in Fiction Since 1900...it was fun to just go down the list and look at the names listed.
So, do you spend a lot of time on naming your characters? What is your approach?