I've just finished AS OCR MEI maths with Decision 1 (got a B), and I loved it. I only took it because it was advised to take with physics (which I ended up hating).
My mental maths skills and a lot of my understanding of higher GCSE maths concepts were very poor, so I struggled with Core 1 at first (I had taken my GCSE maths grade up from a C to an A over the course of my yr 11, so I learnt a lot of it very quickly and not in depth, which I suffered for). But, through practise it begins to make sense. I found Core 2 a lot easier as I was learning it afresh, rather than relying on previous knowledge. And Decision 1 was really simple, although the exam can be very hit and miss (sadly, this year it was a miss for me, I managed to do mocks where I was getting 68/72 and such)
It's a very worthy subject to take, and my advice would be to do as many past papers as you can, ideally all of them. One boy in my class didn't do any, and when it came to a mock about a month before the exam, he got 7/72 on Core 1. The exams are very formulaic, and it's a mix of knowing the content and knowing the exam.