Though it lacks mission mode and online play, Soulcalibur is a great addition to your XBLA library.
That being said, you have to sit down with someone to play a multiplayer match. For some reason (I imagine it's because this is a direct arcade port), online multiplayer was never added. This isn't a very big demerit on the gameplay, but when you consider how many Xbox Live Arcade games have online play, it makes you wonder why it was left out of Soulcalibur. Another disappointing feature is the lack of a mission mode, which was included on the Dreamcast version of the game.
Although those are two glaring faults of the game, it has more than enough to justify the 800 Microsoft Points you'll spend on it. I mentioned the fun (but local) multiplayer earlier. The arcade mode is plenty of fun, and it has several other challenges like time attack mode, and two survival modes (you defeat as many enemies as you can before you die). With those and the added achievements, there is plenty in this version of the game to keep you busy until Soulcalibur IV comes out.