This conversation always seems to get acrimonious around here. And I say "always" because, yes, it comes up a lot.
I will say this: having a knowledgeable and responsive coach who also knows me, how I ride, what my strengths and weaknesses are and who I can easily get in touch with has been huge for my development. Since beginning that relationship in April, things were a little slow to begin with, but by the time cyclocross season came around, there was a major pay-off in results. Hopefully there will be a pay-off for road season next year, but of course that's the future. We'll see.
The thing is, this kind of responsive, personalized coaching by someone who can regularly see you and how you ride and race usually comes with a steep price tag attached. Even more so for coaches with established reputations. I am incredibly fortunate to be paying a pittance to my coach, who does this mostly as a side hobby to help out his friends rather than as a business. Most people don't get that kind of deal (I didn't have that opportunity for many years), and that makes it harder to justify. So that's kind of the question. What level of coaching can you afford, and how much will that benefit you? That second part is the key and it can be pretty hard to predict!