Script Debugger is an essential tool for me.
Ditto.
It has numerous features, but there are several I use all the time which I wouldn't want to do without.
I second all the ones Ed mentions and add:
Find/Replace supports regular expressions.
Built-in Library support.
Built-in text-substitution.
It's scriptable and has it's own script menu complete with available keyboard shortcuts.
You can open a script in BBEdit as an external editor, and all edits show up in the original script.
With the 'Manifest' command you can quickly create an inventory of application and osax dependencies.
Break-Points and the ability to step through scripts line-by-line are invaluable features for writing and debugging scripts.
I bought my first license in 1996 if memory serves and have kept it up-to-date ever since. It has improved my scripting productivity at least tenfold - definitely a good investment.
I haven't tried Smile since OS 9, so I can't really compare the two.
Smile is great and has a number of unique features. I always keep a copy on my system for speed testing, but I do most of my work in Script Debugger.