The default setting has the Mask On/Off Checkbox in the On position so that you're seeing the (black and white) mask rather than the affected image. Probably a bad idea ... I've now updated the effect in the OP above so that the default state is for the mask to be non-visible.
A big improvement Simon. There's a lot of control with very few controls (if that makes sense). Seems a bit snappier on my machine too (late 2009 iMac).
If you've shot dSLR footage flat then simply changing the highlights blend mode to add, shadows to multiply and boosting the highlights saturation gives you a nice filmic look. You can then use the mask controls to change the contrast to taste.
Funnily enough I took a look at your old version a few days ago and created a similar effect but with an additional midtones control. Managed to get a nice separation using three luma key filters but performance was abysmal. Your new keying solution is clever.
This is a complete rebuild of the effect using a some different techniques to streamline things and improve performance. I think it should be quite a bit more responsive now.
The way you're talking about using the blend modes is spot on - good tip for anybody thinking of using this effect for generally enhancing the raw image. (Being able to control exactly where you want the shadows/highlights mask to operate hopefully takes this beyond a standard grading plug-in or what you can achieve with the Color Board.)
But it's well worth trying it out for some more extreme and distinctive looks - in addition to all the options that you can access via the blend modes (and the other controls), the two diffuse controls can really make a difference. Hopefully I'll get time to post a quick tutorial soon.