It's a great course. My favourite AH that I did.
Mechanics is the branch of physics concerning the motion of bodies under the influence of forces. More simply, at AH it's basically just maths applied to real world situations where stuff is moving in some way or another. This made it the most fun subject that I did - you really gain some sense of power when you look at the things you can calculate about real-life situations (how fast/slow a car can travel round a bend to not go off the track, how fast a disc must spin for an object on the disc not to slip, incorporating air resistance into calculations, calculating the bearing that a ship must travel to intersect another ship...).
I found that it was more focused on problem solving than core maths, i.e. more hard questions. In terms of the course content, AH mechanics is much easier to learn than AH maths. Core maths has significantly more material to plough through. But as far as the examination questions go, mechanics is probably more difficult (more problem-solving).
You should look at the course arrangements for the new (and maybe also the old) course. The new one is a distinct subject called 'Mathematics of Mechanics' whereas the old one is branched, with Statistics, under 'Applied Mathematics'. There is very little difference between the new/old courses. I think the new one has a little extra content on torques / moments of inertia. Also look at some past papers for the type of stuff you'll be solving, it's pretty cool.
Another thing you'll probably like to hear is that if you do AH physics and AH maths, a lot of the AH mechanics content will already be quite familiar to you. From this year's exam, I found that someone with only AH physics and maths knowledge could have got 52/100 marks if they managed to get everything right. (This is never going to happen in reality though; mechanics questions are very different to maths/physics questions.)
I highly recommend you do the course! It has significantly improved my maths in general and it's very enjoyable.