iOS applications aren't all toys, far from it. The iPhone screen size is very limiting, but the Retina iPads such as the Air with 2048x1536 resolution are quite usable for a lot of stuff.
The iOS emulation of the EMS Synthi is perhaps the best there is. And just as weird and difficult to get a tune out of as the original.
I use Korg's iOS Ms-20 emulation quite a lot, it gives me something to construct patches on when I've a few minutes and don't want to fire up the real thing and it's also a useful kind of visual patch librarian for the hardware. The emulation isn't quite the same as my hardware but wmulations rarely are.
Inter-app audio makes quite a lot of things possible on an iPad, including multi-tracking into Cubasis or a similar "mini DAW". Nothing like as powerful or user friendly as a modern PC/Mac application and track freezing is pretty much essential, but it's certainly usable. Things like Korg's Gadget are much simpler, but that doesn't mean music can't be made with them.
iOS also MIDI networks very well which opens up all kinds of possibilities, including as a control surface.
As tablets get more powerful I can see them taking over from laptops for a lot of live performance work.