Greenstone3 v3.03 has been released for Windows, GNU/Linux and in Source form (it runs on the Mac no problem; just recompile from source). This release finally contains all the features in Greenstone2, including OAI-PMH and Remote Building support (which are new to this version). It is now very easy to install — just as easy as Greenstone2! This release has been extensively tested.
The principal difference between Greenstone2 and Greenstone3 is the format language. Greenstone3 supports more sophisticated XSLT format statements. Right now they are more complex to use, but developments in the pipeline will make it far easier than Greenstone2 in future. (The only other differences are that Greenstone3 still lacks collaging, and collection/document-level authentication.)
There is one known issue. We forgot to ensure that collections built with Greenstone2 are automatically converted when loaded into GLI under Greenstone3. We will add this facility, but for now a manual conversion program is available here; instructions for its use appear in Greenstone3 for Greenstone2 Users.