HCAP's aptitude tests have been around for a long time and are based on those used by the RAF (the tests take place at RAF Cranwell). They used to be the only independent tests available (i.e. not affiliated with a school so no commercial imperative to pass you if you're below standard). I would certainly recommend taking one. As the test is designed to gauge your ability to learn to become a pilot, rather than assessing how good a pilot you already are, you probably shouldn't do too much prep ahead of undertaking one (they are assessing your potential not your current knowledge level). That said, brushing up on your mental arithmetic wouldn't hurt and there's a handy book you could purchase called Mental Math for Pilots, available from most online pilot shops.