I agree with most of what Susan has to say in the matter of using the product

suppliers sales engineers rather than consultants even though I have been a
consultant for a number of years. Most suppliers are very knowledgeable and
of great service to
their customers but it is important to remember that they are seeking
customers for their products and may on occasion skew their presentation to
serve their own needs. You need to feel comfortable with the vendor that
he/she is serving your needs properly. 

Still consultants can serve a definate purpose where the client does not have
the time to spend sifting fact from fiction regarding product claims and
wants the results as soon as possible relying on the expirience and expertise
of a concerned but unbiased professional. They are a member of your team,
even if only for a short while and can often save you much time and possible
grief. I would only recommed that you request and check their references.
 Among the many consultants in the industry are testing labs which provide
knowledge and services that their client does not currently have and serving
as a referee in the case of conflict. You see consultants are not all that
bad.

Cheers,
J. Fjelstad