I used Entourage for about for months. Fortunately I was smart enough to back up my database on a regular basis. Disaster struck a couple of months ago. It happened while downloading a largeish message (probably close to 1 meg attachment). Entourage crashed and when relaunched told me that the database needed to be rebuilt. Unfortunately neither the standard or advanced rebuild was able to solve the problem. My database was useless. (BTW, my db wasn't excessively large, only around 70MB.) I was hoping that the recent update would be able to resurrect the old db, but alas it told be something to the effect of "Your database probably can't be rebuilt." Since the disaster, I have moved all of my mail back to "Mail" and have been happy once again. I had always missed the ability to grep by mail remotely anyway :) I now feel much safer knowing that my mail is in a standard format.
I'm not suggesting to stop using Entourage if you like it. I have a friend with that happily uses Entourage and has a db of nearly 1.5GB. Just that if you use it, you should be aware of the dangers and make sure to back up your db regularly.