The concept is simple. An easily summoned and retractable space for note-taking while you're working in another application, controllable from the keyboard so as not to interrupt your flow.
The question is whether SlidePad delivers $20-worth of functionality compared to TextEdit, Stickies or Sidenote.
It does the basic job. You can set any key combination to control SlidePad, and decide whether you want it to appear from the left or right of the screen.
You can create multiple named notes (in effect, documents) and cycle through them using keyboard combos or the mouse, and perform all the standard text-editing functions plus a few nice extras like Paste And Match Style, which pastes text in the style already in force at the insertion point, and Jump To Selection, useful when you've scrolled some way from selected text in a long document.
Unfortunately, that's about all it does, and some features are of questionable utility. Is it necessary to have kerning in a note-taking app? And why does the Check Spelling While Typing option miss so many errors? (The document spellcheck works better.)
It's possible to use the mouse instead of keys to bring SlidePad into action, but it can make unwanted appearances when you're working close to the edge of the screen in another app.
SlidePad struggled with long strings – a 22,000-word passage (which didn't cause TextEdit to blink) took about 20 seconds to reappear after another note was visited. This makes it nearly useless as a temporary hold for large chunks of data.
In short, the convenience of keyboard activation is all that SlidePad offers over TextEdit. And is the occasional saved mouse-click worth $20 in these credit-crunched times?