it's a great program for enterprise software that wants to secure (not completely eliminate) the possibility of their code staying in only one domain. The downside is that you have to install the Zend Decoder which may or may not work well with other decoders (ionCube, NuSphere, etc.). So you may limit your audience by encoding the project.
On the plus side, it can offer licensing and expiration items; however, at what cost? They're required to have a loader (which if I recall correctly it's not enabled by default) so they may (or may not) be able to install the loader. in the end, it can be more hassle than it's worth.
There are plenty of software packages out there that don't use Zend Guard (or other obfuscator / encoders) and work out quite well. vBulletin is a prime example of a visible source project but while still being "closed source".
