What can you say about a tool that so intuitively reaches into the depths of the internet to show you every link, every share, and every tweet pointing to the site you are researching. I always joke that you can't hide something from an SEO and it's these tools that make that possible.
Can't say enough nice things about Ahrefs, it quickly became our one stop tool.
I wish when you went to the Ahrefs homepage that it let you do the Site Explorer thing instead of trying to get you to setup one of their campaigns, I usually just want to glance at the data quickly, and having to click over to explorer is just one extra step - that is a nitpick, but it's something I deal with on every use.
Just get it - it's a slam dunk, it's easy and you'll have more data than you'll ever use.
It gives us access to the data we need to quickly figure out how much competition a competitor has for our clients, and compare that to find out how much work we'll have to do and how much time we'll have to spend on the clients site/backlinks.