The positive: Great market coverage outside of the US. Provides a real wow factor for new clients who love to see what their competitors are up to and the need for professional ppc campaigns.
The negative: It does take a little time to get used to the user interface and the data on less popular keywords can be a little patchy. Organic reporting not so great.
Overall: KeywordSpy really is a great tool for monitoring competitor search ads and keywords, helping you to quickly improve your own ppc campaigns.