My favorite feature is the nearly real-time position tracking. Compared to other great programs like MOZ, having the position tracking update accurately once a day is very helpful for reporting. I am also very happy with their site auditor.
Some of the usability and functionality isn't as well thought out as it could be. Once in a campaign, I think a user should be able to access all other services as they pertain to the campaign, as opposed to needing to resubmit information.
Customer support is great, since you can actually call and speak to someone. You can also email or message from the site, and they guarantee a 12 hr window before response.
For position tracking in organic search and the site auditor tool I have been very impressed and satisfied.
SEMRush is great for diagnosing website issues, both technical and user experience, and helping our team prioritize the impact on SEM campaign.