We have managed to get Search Server Express installed on a test Sharepoint machine and we admit that we are a little disappointed. Why? Well, we thought that Search Server would integrate directly with Sharepoint. It doesn't. It basically creates a seperate (Sharepoint-like) site on the machine that is used purely for searching.
This means that the existing Sharepoint integrated search functionality stays as is (from what we see so far). However, the Search service does search the Sharepoint content directly and you can extend it to search things like web sites, file locations and public folders. This is where the real power of the application is going to come into its own.
Imagine setting up Search Server to crawl not only your Sharepoint but also every data on your server automatically! This will be a godsend to finally be able to locate all those documents strewn out across server hard disks.
Apart from this what we like so far about Search Server is the fact that it is easy to configure, simple to add crawling locations as well as perform maintenance tasks like launching a full crawl. The Search Server admin site has plenty of information about the searching status and configuration which was alwasy something that was hidden in Sharepoint.
So initially we were disappointed the it didn't integrate directly with Sharepoint but the more we use it the more we like it. Can't wait for the RTM version now.