So I bit the bullet over the week and bought the filter. From what I had read the filters work by spreading the light around the image, which is different to simply using a low contrast lens I think. And besides that, I don't want to add another lens to my collection when I already have three superb ones.
I started with Ultra Contrast 3. It had good effect but I thought I'd go further so this morning I received the #5.
The filters have an immediate effect of making the image look way more filmic. The reason for this is that really do spread the light around the image - but it's not so simple. They take in more ambient light and thus ambient colour. The result is that you get a much more balanced picture in terms of colour and tonality. These filters really are something magical.
I have been shooting a fair bit with them over the past few days and will be putting together a video, but for now here's a few images:
No filter, Standard iDynamic:
With FilmConvert:
Tiffen Ultra Contrast #5 (no iDynamic):
With FilmConvert and some additional saturation.
Each filter costs just over £100 for the 77mm sizes. I do recommend getting the strongest one (the #5). After further consideration I think the #3 is a better choice because the #5 washes out darker midtones too readily.