I've always liked playing on my mobile devices, even back in the Pocket PC / Windows Mobile / Palm OS days. They revitalize me quite a lot, particularly after a hard, exhausting day.
Playing without physical controls (buttons), only using the touchscreen, is pretty hard. This includes a lot of game types, particularly ones where quick direction changes are needed and / or you can't make the smallest mistake and must be absolutely sure even the smallest movements on the directional pads (D-pad for short) is registered. In games like these, physical D-pad / button feedback is of extreme importance.