One cannot review Spacie without comparing it to WhatSize.
The user interface of Spacie is almost exactly like that of WhatSize. Take a look at them side by side - the similarities are obvious.
UI similarities aside, although Spacie costs only $2 as opposed to WhatSize's $13, Spacie lacks a great many of WhatSize's features.
Whatsize also has the advantage of being a Universal Binary that runs under OS 10.4, as opposed to Spacie's being Intel-only and requiring at least OS 10.5.8. There are still plenty of G4 and G5 owners out there who run Tiger, but Spacie is useless to them.
Overall Spacie is a good first effort, but it has quite a way to go before it can compete with WhatSize. In addition, its near duplication of WhatSize's interface is rather blatant.