I use Ion engines when I want to lazily cruise between the moons of Jool. I like the challenge of trying to maximize my science, TWR, and DV. Of course, this is an exercise in sparsity. Since every Ion engine required 16 1X6 solar panels to operate reliably out near Jool, it becomes difficult to keep TWRs from falling below .06 (Kerbin) with any more than 7800 m/s DV. With .06 TWR, I can burn 500 m/s in about 13 minutes, which isn't too bad. Scaling up the vessel while maintaining the same panel to engine ratio opens up marginally more payload space, but at some point this is eaten up by the requisite reaction wheels.