What a book doctor or reading service can do is give you completely impartial feedback. They're not your friend, so they don't worry if they hurt your feelings when they reveal your novel is less than perfect. And their feedback will hurt if it's anything less than ecstatic. It's one of the reasons I personally avoid book doctoring as I want to be honest without upsetting people.
They should be experienced writers, editors or readers (you hope - see later on) so can give advice on the novel as a whole based on both their own writing and draft novels they've seen in the past. Because of their experience they should be able to pin-point problems and suggest solutions.
I used a book doctor on my first novel, Adultery for Beginners. The report identified two problems: 1) it was set in the countryside and, at the time of foot and mouth, they said editors were not buying country based novels and 2) it was written from four viewpoints and three of them were weak. The report suggested beefing up the weaker three and made suggestions as to how the plot might be changed to do this.
I did change the setting to a town - I'm not sure if their assessment was true as plenty of novels before and since have been set in the country. I took exception to the second point, thinking that the suggestions for 'improving' the plot were trite and cliched. However, after a long period of sulking, I did recognise that, while their solutions were wrong - for me - they had pin-pointed a problem. I re-wrote the book from one character's viewpoint, and that version was published.
I am concerned about who actually offers the feedback. If they are a writer or editor of your sort of novel then they should be able to offer useful feedback. A writer or editor in a different area may not understand the market for your genre. I also suspect that a lot of readers are unpublished MA Creative Writing graduates. I say this because after I graduated I received several round robin letters from companies recruiting readers via the creative writing department. So if you can, check out who is actually going to be doing the reading.
I don't think I would have been published without the report, because no one had ever given feedback about the weakness of the other voices which pushed me re-write. So, I'd say it IS worth the money, if you feel confident about the reader and their feedback and can make it work for you.