First, you need to know SEO. Not just the basic of on-page, keywords and search volume, but stuff like Trust Flow, backlink spam score, LSI keywords and beyond.
If you have just started out doing SEO and keyword research, then stick to free tools like Google Adwords Keyword Tool. SEMrush is for the advanced users.
It has everything you need and a little bit more. Whether you want keyword competitiveness, backlink profiles, competitor backlink profiles, related keywords, search trends, gaps in your keywords, your most popular content, your most linked content, and ditto for competitors, you will get it all.
What you do with it, is the absolute key.
The best way to start with SEMrush is to use the free version for some time. If you have already used the free version, and feel you can make use of it big time, then by all means go ahead and buy it.
FREE Version TIP
By the way when you use the free version, after a while a pop-up pops up saying you need to verify email blahety blahety blahety… when you click on this all pervasive pop-up it takes you to the check-out page where you need to pay, or at least give your credit card details for the free trial. The tip is that you just need to click anywhere outside the pop-up and you can return to do you free stuff on SEMrush. Just keep doing this every time the pop-up pops up, and you will be able to get your work done.
If you are serious about SEO and keywords and backlinks and such like, then by all means buy it. If you are able to use it well, stay with it. SEMrush well used can be a lethal advantage for any Internet Marketer.