The Share client only accesses spaces under the "Company Home / Sites" root space. Although these spaces are fully accessible from the Explorer client, we strongly recommended that you do not do so, as Share doesn't yet cover all functionality (as Mark said) and may encounter "unexpected" data.
Apart from the much shallower learning curve of Freemarker/JavaScript within Share, the other advantage of the new client is that it can be run on a completely separate Tomcat instance to the main Repository (to which the Explorer client is strongly tied). This means you can firewall-off access to the Repository for extra security.
Thanks,
Mike