Something to note about solo overall is this
1. Solo traffic is completely different beast altogether. People who got crappy results (assuming good vendors) don't understand this logic. The page MUST be specially customized for solo traffic
2. If you are using a squeeze page provided by an mlm that has been online for more than 3 months, more than likely you're using an saturated squeeze page. ALL mlm offers MUST use a customized squeeze page. Any good vendor will tell you that.
3. Avoid solo ad providers who guarantee opt ins. Run as far as you can. NO ONE is able to guarantee opt ins. More than likely they're using a bot or fake traffic.
4. Don't be cheapskate with solo traffic. 20c per click does definitely sound tempting when the industry standard is 50-70c. Any credible vendor will never be selling at that price. Which brings back the point of fake clicks.
5. When looking out for testimonials, keep a look out for SALES testimonials. 'omg 70 opt in rate" doesn't look very appealing, especially considering if his other testimonials had 0 sales combined.
There's really so many things to note and I'm coming from a solo ad background, and I'm a seller myself, there might have stuff I left out, and if you've got questions about this industry, or how to effectively use solos, let me know and I'd be glad to help