I purchased the first verison of this, because I figured at $5 it was worth it. IIRC, that was when Leopard was at version 10.5.2.
Every new version of Leopard requires a new version of iQuarantineX, which makes sense to me. The upgrade to version 10.5.3 of iQuarantineX was free, but with version 10.5.4 the developer wants another $5.
At the time I purchased iQuarantineX there was no indication that my $5 would only last a couple of versions of OS X. If I was to pay another $5 for version 10.5.4, there's no mention of when I'd have to fork over another $5 for a future upgrade (but I'm guessing it wouldn't be long). So I'm not going to purchase the new version, and I'll learn to live with Leopard's occasional reminder that I downloaded an app from the Internet.
In terms of functionality, iQuarantineX does what it says it will do, and it is dead simple to use. In terms of value, it's not worth it. A good app ruined by a bad pricing strategy.