I am also a Fiverr seller since the last few months and have got around 250 positive reviews by now and increasing everyday. I would not say that Fiverr is bad for your site. There are a lot of great gigs on Fiverr which are real value for the money. You just need to select and use the right gig for the right place be it for SEO or other work. The problem with Fiverr is that some of the top SEO providers on Fiverr just lie in their gig description. But that is where you need to know about SEO yourself so that you do not go by their words only. There are very good and honest sellers on Fiverr too though probably less than 10% of the SEO providers there. I personally give more importance to ethics over money due to which I reject a lot of requests if I feel that it might not fulfill the buyer's requirements. I just tell everything honestly and work on ethics due to which I at times lose some buyers in the short term but get long term and big clients who trust me even more. So, you just need to find the right buyer. Reviews give a good idea for the initial filtration but not the full idea. Humans at times tend to go for "more" "quantity" rather than "quality". So, a gig promising a 50,000 comment links as Google penguin panda hummingbird xyz animal passed would often get more reviews and buyers compared to a genuine gig informing about 20-30 manual comments. But any real SEOer would tell you that the first gig is just a SB blast which would probably result in negative SEO for your site in the present date while the second gig has some chances of providing better results. Similarly, a gig promising 1000 social bookmarks as awesome for money site might look attractive at an initial glance but a quality gig providing 20-30 high quality social bookmarks from top bookmarking sites would ideally give better results. Again, many services have their use at different tiers if you know to use it properly. High quality gradual manually built links are good for Tier 1 as a thumblr rule and the blasts (especially the big ones) are preferable for Tier 2 or 3 onwards.
So, read the forums and understand SEO yourself before purchasing XYZ service out of the blue be it on Fiverr, Seoclerk or anywhere else. With time, you will know to detect the lies easily and be able to filter out services based on not only reviews but also content of the service. But except for a few selected gigs where you only purchase the resources (such as PBN or expired Web 2.0 names or high quality bookmarks or video creation service etc.), try not to purchase SEO services directly for your money site. Even if someone says PBN links or XYZ quality links, remember that those are all public by now and so, has more risks of being penalized.
