The SATA controller blew up on mine (also an ALS / Rev. B 17") last year. Otherwise, it's been fine. I think speedwise, it's starting to show its age. I'm not in a huge hurry to upgrade, although I'd probably have been more inclined to do so if the SATA issue hadn't happened (as it is, it'll probably be hard to re-sell an iMac that needs its drive to be in an external enclosure...).
These days, I generally surf and play on my Eee, but my iMac maintains my 2 gigs of e-mail, my iPhoto and iTunes libraries, manages all my iPods and iPhone, etc. So it still receives regular use.