I was having CPU usage issues. I do have quite a few notes and am seeing quite a bit of CPU usage and the corresponding reduction in how long my MacBook Pro 13 can run on battery -- to me this is a critical issue. Perhaps optimization will help this soon.
After a second restart after the 10.5.7 upgrade, CPU usage dropped substantially. Perhaps, there is an issue between the Helper(s) and the main Evernote when software upgrades happen since they seem to be happening within the main application.