The original playstation 2 is not a very sought after system now with the slim line and the playstation 3. Something a lot of people don't think about is durability and the fact that the original playstation 2 is about as durable as you can get. The size may make it harder to pack up and take with you but it is (much like the original psp) a solid brick. How hard is it to break one of those?
Also there are very few problems with the original playstation except, my common problem, the disc read error and that can be fixed. I know several people who have complained about their slim line ps2s getting over heated and then not working because they left it on while doing other things. I usually try to turn mine off but if I'm playing a game and have not gotten to a save point and HAVE to do something or go somewhere I don't worry about leaving it on. I also think that's a common problem with the PS3.
It's easier to mod. I got a flip top for mine with swap magic to play my japanese Kingdom Hearts 2 Final mix+ and installing it was a breeze. Of course it depends on the model you have mine is a V10 and the cable that holds the top to the reset/eject switch is not there so when you pull the top off you don't have to worry about breaking it. I didn't think you would have to have one for the slim line since it is already a flip top but I've tried it with my swap magic and it didn't work.
Last but not least the price. An original Playstation 2 is MUCH cheaper then a slim line or a PS3. You can get a PS2 with controllers, memory cards and accessories for 200. A new slim line is 130 at Walmart for just the system and a controller. The PS3 is 400 for the system and a controller. What price sounds better to you?
I know there are those people who like having the newest systems and of course who have PS3 games they want to play but if you just want to play playstation and playstation 2 games why spend the money on the slim line? The original will last you longer and it withstands a lot I had a toddler stand on mine and my dogs have stood on it. Nothing has happened and it's going on 7 years old. My money was well invested.