Prince of Tennis wasn't bad if you were a tennis fan. At the start, you had a the kid who is better than everyone because he has a pro father who trains him. Okay, cool, that sorta makes sense. I'm on board.
Then the other main high school characters start exhibiting shonen-esque special tennis powers. At this point I'm like....what is going on in this show...okay its only once in awhile, its not awful. Later in the show, it just goes right out the window. There isn't realistic tennis anymore or anything that's a semblance of being fair for anyone that is not these crazy ass kids who apparently all have magical tennis powers. It is no longer about playing the game, just trying to one up each other with who's special power is better. It's like the NBA Jam of tennis thrown right in the middle of what was originally a pretty down to earth program.
For the people that like Dragonball better than DBZ, it is similar. Like in DB, you have a lot of people that are similar power level, even humans! There are martial arts tournaments that they compete in and actually have challenge. DBZ rolls around and it's "LAWLHUMANS We're aliens bitches".