I see no harm in using the reviews captured as verbatim or screenshot image... If someone has provided a review on a product online then you can expect that to be leveraged given that its already in the public domain... I would also consider adding a reference to the source as a footnote... That said you should definitely use a disclaimer, especially if you are considering using ads e.g. FB, adwords, bing etc as that is required i.e.
Testimonials found on this site are examples of what we have done for other clients, and what some of our clients have said about us. However, we cannot guarantee the results in any case. Your results may vary and every situation is different. No compensation was provided for these testimonials.
As well as a general disclaimer e.g. You should not use this information as a substitute for help from a licensed professional. Individual results may vary.
Finally as above, if you can contact them then I would do that... Not only ask them - but also ask for a detailed review as that may help you out even more...
Hope that helps!