Since i'm playing a lot on my notebook i sometimes was forced (by various circumstances) to play on the touchpad. While it is definitely possible and not as a pain as it might seem, it is a bit harder than using the mouse. I found out that with some practice i was able to play against Hard AI quite well (won most of the time) and even the campaing is playable (most missions on Medium though, many Hard achievements are **much** harder to get when using the touchpad).

The key to succes is to use the keyboard as much as possible. Everything should be mapped to a  group - units, workers, production buildings. Use space to get to the action immediatelly when something happens. With some training, you should be ok. I wouldn't however attempt to play against people in 1v1 quickmatch, especially if you are ranked gold or higher.