Its worth playing, and they arent related unless they have a 2 in the title. Given the only "2" coming out in the west is Tales of Xillia 2, only Symphonia and Symphonia dawn of the new world are related.
But, not all Tales games are worth playing. There are some really good ones and some well... not so good.
I recommend:
Tales of Eternia on the PSP or PS1 (goes by the name of Tales of Destiny 2 on the PS1, but its unrelated to destiny, wich has its own sequal only released in Japan)
Tales of Symphonia on the GC or PS3
Tales of the Abyss on PS2 or 3DS
Tales of Vesperia on the 360
Those 4 are the cream of the crop. Phantasia and Destiny were decent in their own time, but nowadays are way too dated and the remakes never left Japan.
The ones to avoid:
Tales of Legendia
Tales of Graces
Tales of the World: Radiant mythology
People say Tales of Symphonia dawn of the new world is to be avoided aswell, but i have not played it so i dont know.
For reference i thought the lastest, Tales of Xillia was ok. I didnt feel like it reached the hights of the other 4 i mentioned mainly because the developers decided to split the story in 2, wich resulted in lukewarm storytelling in my opinion. But this doesnt happen with Xillia 2 and hopefully Tales of Zestiria, so im hoping better things from those two.
The Vita tales games, wich are remakes of the often not very good DS Tales, might be worth checking out if they get released. I know Innocence has a somewhat childish cast, but Hearts seems more in league with the Xillia age range.
