It is never good to rely bluntly on career advice by an agency. You should always consider such advices carefully and critically as in the end you are the one most affected by such advice. As for how much you should rely on such advices, it is affected by many factors such as the source of the advice, what the advice is about etc.
The advice can be generic or specific to you as in given by a career consultant after several tests, face to face discussions etc.
In case of generic advices you should consider first, whether they actually apply to your situation. There are tons of such advices available on the internet for free as well as on websites that require a subscription fee. No one can realistically go through even a portion of it. You should decide first what exactly are you looking for and search for it. Use multiple sources and web sites. Keep track of the relevant pages by book marking them. There some web sites that are career oriented. Make a habit of visiting them once in a while. You'll often find things that are helpful to you but those you never would have thought to search for on your own. Your objective in searching generic advices should be to find new tips, information and / or methods that can be helpful to you. As for how much you should rely on; you have to make up your own mind. If you are in doubt get second opinions.
As for professional and personalized advices; their reliability is heavily dependent on the source of the advice. Make sure that the advisers have proper credentials and their knowledge and experience is relevant to your career line. If you are in anyway uncomfortable about some or all of their advices don't hesitate to questions them. No matter their status or qualification, when all is said and done it is your career on the line and not theirs.
What the career advice is about is also important when you are deciding whether to rely on it or not. For example when you are deciding which career line to choose, it is a very personal decision and no one option is completely wrong as there are opportunities in all career lines if you have aptitude and inclination. The career advice in this situation will be helpful to you by providing you information about various career lines, and you should not rely bluntly on a certain career advice. Suppose you have to choose between two similar jobs within different companies. The decision would be more dependent on which company offer better compensations overall or which one has better growth prospects. In this situation the individual factors are of less importance as you would be doing similar things in both companies. The advice of a consultant should be considered much more carefully in this case.
In general you should realize that a qualified career adviser would know more about the job market and industry but you'll know more about yourself and what you really want; therefore you should rely more; not bluntly or completely but more; on the career advice when it is concerned with matters of industry and job market and less when it is concerned with thing that are personal to you. For example you can rely on career advice about which company will pay more or would be better for you in long run but rely on your own judgments when deciding whether to go into a technology oriented career or a people oriented career. You should take career advice for deciding methods and not objectives.
Also personalized career advices are suitable for some situations and generic advices are for others. In general generic advices are good enough for interview tips, resume building, interview preparation etc. You should seriously consider taking personalized career advices when facing career or job change, while choosing a career profile etc. As these are more suited for personalized advices and / or have long term impact on ones career. In every case though use your own judgment as well, as in the end you will be the one most affected.