I don't know about the Dreamcast game, but you can get the PC version, and there are still servers for it. The console versions (also available for Gamecube) have an offline mode, since they obviously can't expect everyone to pay for internet access. I don't think the PC version has Offline mode, but you can always create your own private "room" and play without anyone else. As for the game itself, it's pretty fun. Combat is all action-based with ATK/DEF/accuracy stuff thrown in the mix. It's easy to pick up, and your controls can be customized to any way you like. If you're not familiar with the series, it has a sci-fi type story with some magic thrown in. You'll be fighting things from animals to monsters to robots, and the bosses are great. There are four modes of difficulty, and enough quests and randomization of dungeons/drops to keep you playing for a long time.
So basically, if you want to go online, get the PC version. If you want to only play by yourself or with friends (four player split-screen!), get the Dreamcast or Gamecube version. I recommend the Gamecube one, personally (look for "Episode 1 & 2"), as I believe there are still private PSO servers set up in case you decide you want online play after all.