I also use post-it notes - I have a small supply of the little ones stuck on the inside of my Palm cover. I also carry a small Notepad and I have a supply of post-it notes stuck on the insider cover of that too. The thing with Diddlebug is you have to reach for a stylus. If I happen to have a pen in my hand, a post-it note is easier. It's the same process to transcribe the note into Grafitti (actually easier to put it where you want from the post-it), so what difference whether it's in Diddlebug or on a yellow sticky?
Slap! is one step more removed from quick writing - therefore not efficient for me. It's a nice concept but I either have to take the time to grafitti in a form suitable for transferring into my Palm apps. (and yet have to edit in the target app.), or edit the Slap! notes later (Processing). If I was going to take the time to grafitti at first instance, I might as well go straight into the Palm apps or use DayNotez or MemoPad with MegaWiki.