It's alright. It has a good amount of positives, and some negatives to go along with it.
I'll get the bad stuff out of the way. This game COMPLETELY favors the button masher. Sometimes you can't go into a single online game and not play somebody using Mitsurugi or Kilik, and randomly hitting buttons. One other thing I don't like about this game is that there is a lack of single player modes. The only single player modes are story, arcade, and tower of lost souls.
The lamest thing about story mode is that each story mode takes around five minutes to complete for each character. Arcade is pretty much your standard arcade mode for any other fighter. Tower of lost souls is fun for a while, but then it just gets ridiculously hard. There are 60 floors total, and I'm stuck on 21-23.
Now for the good things about the game. The character creation is HUGE. There is a lot to unlock and you have a ton of stuff to work with. One thing to note is that you don't get everything from the start. Some items are found in the tower of lost souls, and some are unlocked by getting honors, which are an in-game achievement system.
Even though the online has many button mashers, that's not to say that it's bad. When you get a good game going without button mashers, it's pretty fun. It also doesn't lag like all the other online fighting games their are.
Cliffnotes: Soul Calibur IV is a good fighter, and if you have liked the previous games, then you will like this one. The goods overshadow the bads, but not by much. I don't regret spending $60 on it, but you can get a better game for your money.
(I have way too much time on my hands)