I've been on the beta trial since December and it adds a whole new world to PS2 games. It is so different playing other people instead of AIs. Revenge is something AIs just don't understand!
It's really easy to install. I hooked mine up to my DSL router in about 15 minutes. If you can, get your self a USB keyboard and plug it in for the sign up process as the controller is not the most friendly way to type names and addesses to register. Also make sure you write down your PS serial number BEFORE you insert the adaptor as it covers the serial number up.
Hardware Online Arena is simple but loads of fun and there is always someone in OZ or NZ to play. I have also tried Destruction Derby Arenas and it is furious online. I spent a whole day playing it when it first came out.
The online lobby idea works well especially if you have a head set so you can talk to others about setting the game up. The servers are lag free and NZ/OZ are the only ones to be moderated, I think. It is mostly hands off and it works well.
What you should gather from what I have said above is that PlayStation 2 Net is the next stage in console games. Grab the starter pack, get broadband, get playing!