A bigger thing to keep in mind though is the feel you have when communicating with a potential counselor - is this someone I can work with? Will I respect what they have to say? Do they seem to be listening to me? Do they understand what I want to accomplish and why? I think that clients, and often counselors themselves, are interested in the sense of "fit" they feel with each other, as the applications they are discussing together are of great importance and clarity and focus are needed to be successful.
However, beyond the flexibility of the scheduling, and beyond the comfort level you have with a prospective counselor, one thing stands above everything else.
Has your prospective counselor helped people gain admission to your top choice program(s) before?
If they have, great. Ask questions. What years, and how many people? Ask them if there may be anything different about your situation (note: there definitely will be).
If they haven't, then naturally you are accepting some degree of risk if you elect to work with them. This, to me, is simple common sense. What have they done? If you are aiming for a particular top MBA and they have a lot of experience with other top programs, then this could be alright. But what if they have little or no experience helping people get into MBA programs at all? Maybe in such a case their selling points include transferable skills that they feel will help you with your application, such as their own admissions experience or their communication skills gained from a different kind of job. This may work or it may not, and the risk you assume is based on the idea that they will be able to figure things out and adapt well to this new situation as you work on your applications.
The key here is to look around, and talk directly to those potential counselors about their experience and your needs. Choose the one that offers the best fit for you, and don't settle at any stage of the process. This may take a bit of time - but in the end it will be worth it.