I just switched from Yahoo Desktop Search to the X1 free client. Doesn't look any different to me and has the same problems.
One of the big ones that many people have been complaining about in the YDS and apparently, the X1 version is the locking of the Outlook PST file. This causes problems with backing up the file.
I shut down Outlook tonight to test if X! has the same problem as YDS. I run the free MS PST backup add-in which I have set to run a backup every 1 day (when/if I shut down Outlook. Sadly, I got the same error as I got in YDS:
Cannot copy Outlook: The process cannot access the file because another process has locked a portion of the file.
Since I have X1 set to index Outlook every 12 hours (I don't get much mail on Outlook), I ask why X1 can't release its hold on Outlook when it is not actively indexing the file? This would be a simple solution to the whole problem!