I just bought an IMAC 600 (the crt IMAC), which has the Harmon Kardon internal speakers, and I'm wondering about adding the iSub.
Does anyone have experience with the iSub?
I'd pick one up on EBAY if it's worth buying.
I know for a fact that the internal speakers of all the slot-loading iMacs don't sound too bad, and just by adding a subwoofer, can greatly increase the overall quality of your sound.
Unfortunately, the iSub usually isn't cheap (between $120-$200 on average), so I wouldn't recommend buying one for the same price as a 5.1 surround system with a even better subwoofer than the iSub included.
I had a Flat-Panel iMac before I got my G5. I used the Apple Pro speakers (made by Harman Kardon) that came with the iMac; since the Apple Pros don't really have any low-end, I added an iSub to provide bass. It was a great setup.
Overall I highly recommend the iSub, the only caveat being that sometimes USB audio devices (not just the iSub, but any speaker system connecting through a USB port) have minor issues e.g. the computer sometimes (emphasis on sometimes) forgets the iSub is there etc. and you have to unplug it / reconnect it to get the computer to recognize it. But this shouldn't stop you from getting an iSub, if it's your heart's desire - it's very good...