It's really not horrible or anything, though it can be exhausting. You'll do a lot of tests--most of them will be rather easy, because they'll be testing things that most people can do in order to find out whether you've got a deficit in that area. For example, I had to put pegs into holes, squeeze a hand-grip tester, put shapes into their matching slots without looking. There were also some personality type tests, and an IQ test, and some tests that were a lot like puzzles--asking me to trace the patterns in a series of colored cards or match shapes and colors into categories. They tested auditory perception by having me listen to nonsense syllables and write them down. Some tests were frustrating; others were almost like games. A few were boring. I did tests of basic reading and math--like reading words out loud, or answering things like how much change you would get back from $20 if you bought some things; very simple questions that most people can probably do. Other tests just got harder until I couldn't do them anymore, but they weren't really confusing. I think many of them are structured to start out easy and keep getting gradually harder until nobody can do them. You shouldn't expect to finish those sorts of tests; you're not supposed to (if you do it's called "hitting a ceiling" and means the test is too easy).

So yeah, not a big deal. Don't be afraid to ask them to clarify instructions, or to ask to take a break if you need one. If you start to get overwhelmed, take a break; the test is no good if you can't put out honest effort when you take it.