Are these wide angle adapters any good?
 

The 72mm filter like lense adapters like the one below.
http://cgi.ebay.com/ws/eBayISAPI.dll...ype=osi_widget

Is it a viable cheap alternative to the expensive wide angle canon lense?

With optics you generally get what you pay for, and cheap glass has corresponding performance. Given the price point it is unlikely that it will match the performance of the Canon glass, however, it may be acceptable for your artistic intent. Note also that it appears to have a significantly different factor (0.5x vs. 0.75x).