I'll give you my experience.
I actually found the LNAT to be quite enjoyable. Well, almost enjoyable, lets say. It's honestly not that bad and there's quite a few interesting passages you're going to be given. Your task is usually to understand exactly what the writer is trying to say with the passage. This may be hard for some people and easier for others, and it's not really something you can prepare for in just a few weeks or even a few months (LNAT themselves state that this is not a test you can revise for). I did practically no revision and scored above the cohort average for this year. The only form of 'preparation' you can really do is practice tests on the LNAT website.
In essence, the test is there to test your aptitude for a law degree. There's obviously a lot of reading involved in law, and therefore some of the most competitive universities have chosen to use this test as a way to filter out unsuitable candidates. If Law is the right degree for you, then you'll probably do quite well in this test. If you take the test and end up getting a poor score, it's likely that Law might not be the degree for you, given that you'll be expected to do a lot of complicated reading (which you'll have to understand) throughout your degree.