I don't know about SC I or III, but I own SC II and I love it to death. The regular arcade mode isn't anything particularly fantastic as far as storyline goes. However, seeing the characters interact is always a joy for me (destined battles). Weaponmaster is a fairly decent storyline. It can be a bit redundant, but the end is good.
For me, the big thrill was the visuals. The characters, the colors, the places... beautiful.
The gameplay can get to be a one-button bash-fest at times. However, if you get far enough in the game you start having to use your brain, especially in Weaponmaster where there's different things you have to do to win. Kilik, Raphael, and Talim all have some annoyingly inescapable moves that you can pretty much use to pwn anyone. Yeah, yeah, I used Talim constantly. But that's besides the point.
My point is that Soul Calibur II does not take fighting arcade games to a whole new level or anything, but playing it is an enjoyable experience nonetheless. If you like it, buy it: hours worth of entertainment.