As Director of SEO at Blue Magnet Interactive, I share my views regarding SEMRush.
The Pros of SEMRush are pretty easy.
1. It is fairly easy to use. The interface makes it fairly clear how to get what you want.
2. The keyword data is great and was a lifesaver after Google's keyword tool started obfuscating search volume.
3. Setting up projects to help monitor keyword visibility, brand mentions, as well as the social media monitoring, has made a lot of the manual tasks I used to do obsolete.
4. Recently, they've added an SEO content template tool which replaces a process I used to do to develop a set of semantic keywords to include for any given keyword topic. After just a month of using it, I am very excited about the results and hope the success continues.
The Cons of SEMRush are bit abstract.
1. It is an evolving tool which means how you got information once might not be how you always get that information. Since it is web-based, you are stuck with whatever updates they make.
2. The backlink data they show is really weak. In fact, beyond using SEMRush backlink information for very quick audits, I don't find it that useful.
3. I don't think the keyword difficulty score is all that meaningful either. It seems a keyword is either really difficult to rank for or not difficult at all. There is very little middle ground in the scoring, at least from the keyword sets I tend to work with.