I love the IIGS mainly for the ports of Sierra's AGI games. The IIGS versions were the only ones that did actually have sound fx (woooooooooooow!!)
and the music used a few instruments too. I always wondered why they didn't use it for the Amiga versions (since the amiga was indeed capable of it). But I think the IIGS versions were made after the Amiga versions, and thus uses a better version of AGI.