To Ivanovitch: I run TuneBar (2.7.2) on a dual monitor set up and it seems fine. This is on a DP G5, not an Intel. I also have it installed on an MacBook Pro, but I haven't used it there other than to start it up, set my preferences, and make sure that it worked.
Can you be more descriptive about the instability you see?
One thing I did notice is that if you use the standard TuneBar notification (instead of growl), then it does assume the left side monitor isn't there (it slides the notification off the right monitor and onto the left monitor). However, that's not a really a problem, and if you switch to using Growl notification, then it's not an issue at all.
In terms of the CPU issues I saw with earlier versions, that seems to be resolved. I still see CPU usage spike up, but that's to be expected. The constant CPU usage I saw before isn't happening.