I happen to have a vanity plate and based on my experiences probably would not do it over again. For me, it was part of the pre-delivery experience and a way to build anticipation for that first drive. Once on the car experiences have varied.
Often I get good comments and thumbs up on the plate (esp. at PCA events), but I've had some really trying moments as well. For instance, the early twenty-something girl passenger behind me one day in stop and go traffic that hung out the window & kept screeching my plate wording at the top of her lungs for a full frickin' 10 minutes! I think she must have kept going for a lot longer after our roads diverged, 'cause I kept hearing it in my sleep that night.
Then there was the minivan that became the lamprey to my shark in the high speed lane at uncomfortable speeds...