I owned one of these when i had a ps2 for 2 reasons.
The first reason is in order to play games online. Mainly I used it for playing pro evolution soccer 6 in the pes leagues and had a really good time doing so. Its far more fun to play online against real people than it is to play against computer apponents.
The second use is to attach an IDE hard disk to it in order to (officially download socom maps to) backup my original game disks in order to play the games of the hard drive using a peice of software called hd advance 3. This was especially useful as the laser on my ps2 was faulty and games like GTA would stutter really badly whilst playing.
The adapter came with a disk called network access which was a little confusing as it made out that you needed to use it in order to play any game on the web, this turned out to be incorrect as I could play PES even if the disk wasn't used. It turned out the disk was to access sony's network which was rather dull and I never used it.
If you have an old phat ps2 then I would highly recommend getting this for the online play alone.
This product was a bit of a waste of time for me as i didn't realise that if you had a slim PS2 you did not need this prouct. So if you have a slim ps2 just order a network access disc from somehwere and get an enthernet cable and connect that up to an internet access point.
The good thing about this product is that it comes with the network access disc anyway. When you are hooked up to the internet it is great to play against people from all over the world. Much funner than playing offline because you no this way you are actually playing someone.