One major downside I think of this software is that they hard-coded their Amazon Associates ID into the application. The down side being if you follow the "buy" links, they get a kick back. Additionally, paying $40, then knowing they drive all this traffic to Amazon with their Associates ID kind of irks me. I was very disappointed when I discovered I could not change this Associate ID's value ( as other similar applications allow one to do ).
A couple of other things that bother me, is that duplicate scans make duplicate entries, then deleting the extra entry from a long list, the window pops back to the start of the list ( leave the pointer were I am ).
DVD lookups fail on non-US releases ( lots of Region 1,2,3 dvd's here ). The export of the library is a simple tab delimited file ( XML or HTML would be nice here ).
The ability to scan barcodes with an iSight is cool. Scanning large collections goes pretty fast.
Another review at http://arstechnica.com/reviews/apps/delicious-library.ars (http://arstechnica.com/reviews/apps/delicious-library.ars) covers some pros and cons and competition as well.