They are kind of a good gauge, but not 100% Both in regard to the trustworthiness of the client and the host.
I had just returned to the hosting business after being gone for 5 years. I had a few negative reviews prior to leaving the hosting business. Part of the reason, and this really taught me huge lesson, is I ended up having a personal tragedy in 2004/2005 and my business suffered a little bit.
To make a long story short, I got a few negative reviews. When I came back I honestly wished I could start fresh. However I felt that you can't run from your past, just deal with it the best and move on.
I don't care how great a web host you are, you are always going to have that one or two customers that no matter what you do are going to have issues with you.
I had over 600 clients at one point and only 2 left a negative review.
My personal tragedy was nothing I did intentionally.
I will write post explaining what happened and will come back and link to it. I don't want to take this thread to far off topic.
Trusting reviews, there is a lot of conditions that they can't be the end all trust. However it is good to review them to look for signs. You see a run of "This place stink" that is usually a good sign that something is going on. On the other hand if every single review gives 5 stars, they are the complete best, I am skeptical. Again depending how many there are, how old the reviews are, etc.