Having a personal plate can come in for unwarranted criticism. I see it most often when the plate is the user's name or similar to it. To a point, yes this is a bit vain but even then I don't feel the need to have a go at it.
Other plates may be some sort of jokey thing that relates to an aspect of their character , interests or legends about them.
I don't have a personal plate, but recently I had a browse out of curiosity as to what kind of things are available and how much they cost. The best one I saw was MOG 9X. I thought that one was just magic and if I had the money to fling around I'd snap it up. Since I identify as a cat the MOG bit suits and the 9X relates to cats' nine lives ( in my head ). It was two and a half grand listed.
But it could have been a lot more, if it meant more to enough people. If its worth it to you at the price in relation to your free cash, then go for it.