I just went through half of the tutorials; this program is amazing!
I used to run Derive on an HP200LX and MathPad on my PalmOS devices. SpaceTime on the iPhone is more powerful, faster, and the interface is much slicker. Being able to pan/zoom the 3D graphs with multitouch is fun.
HP 48s, 200lx, USR Pilot 1k, Palm 3, V, M100, Handspring Visor, Sony N610c, T615c, T655c, HP iPAQ 1910, 2215, Palm T3, HP hx4705, Dell X50v, HP rx1955, Apple iPhone, 3G, 3GS, 4, Samsung Galaxy Nexus