Just to add some variety to the opinions, I'll be studying Maths at university this September and I'd still recommend Psychology. Got an A, but really would've liked an A*, but my time management wasn't great so I didn't do as well in the second paper.
Anyway, with enough preparation it is possible to do very well in this subject. Not only that but for the most part it is interesting, so when it comes down to revising it doesn't seem as laborious as it can do in other subjects. AS is pretty easy, but A2 is where the hard work really starts. On the AQA A specification, in PSYA3 there were ~21 essays (24 marks) that you had to learn content for, to which only 3 came up, so you have to prepare in advance. With that in mind, if you can sufficiently prepare for them and memorise the essays and have them marked so you have a rough idea of what grade you're on and where you can improve, you stand a good chance at getting a great grade.
So I'd recommend choosing it if you've got a decent memory, because the subject was thoroughly enjoyable at times.