I'm seriously considering moving my blog to Drupal 5 Beta 1. It seems stable enough, and the performance (especially with Aggressive Caching) blows Drupal 4.7.4 out of the water. That's becoming a pretty serious factor for me, as the insane onslaught of comment spam just keeps bringing my server to its knees (I'm looking at YOU, Apkakkallli!)
There are only a handful of modules that I use that don't have Drupal 5 versions. Of those, I could probably limp along without them until D5 versions are available.(computed field, img_assist, recent blocks, TinyMCE...)
Stephen's already running a public site on D5, and it seems to be going OK. I just did a test upgrade on my desktop box, and it basically went OK. Well, nothing blew up so fatally that I'd lose sleep over it. And Garland sure is a nice theme...