Customer review sites can be very useful when researching during a decision-making process but how much credence should be attributed to anonymous reviews?
Frustration and injustice are strong motivators which lead to a high volume of reviews being negative. It's our method of 'pay back' when we feel that we've been treated badly; we want the usurpers to take notice of our power as customers by warning others to avoid or tread carefully.
So it follows that when reviews are overwhelmingly positive, it's worth digging deeper starting with the reviewers' other reviews. This is obviously impossible with anonymous or 'Guest' reviewers whose reports may be valid but are unverifiable.
Many anonymous reviews are credible but, in general, are often be treated with a little suspicion an have less influence on decisions. It's easy for companies to fake their own good reviews or for their competitors to submit negative reviews - it doubtless happens - and the reviews may not be anonymous either, which is why we should take a quick glance at reviewers other reviews/opinions when building as complete a picture as we can.
To summarise, anonymous reviews may be better than nothing - but not much.