Several web sites claim the thermal conductivity (of a nominal 1 meter thickness remember) of Depron is around 0.035 W/mK. That would mean a 6mm thickness would have a U-Value of about 5.8 W/m^2K. To put it in context the building regs for walls and floors require more like 0.2 to 0.35 ish so 6mm Depron doesn't reall add a whole lot of insulation in real terms.
Having said that if 6mm is all you can really afford to raise the floor levels then go for it. Some insulation is better than none. It would be much better to lift the boards and put 100-150mm of something else between the joists.
I look at all insulation this way...
Aerogel is the (or one of the) worlds best and most expensive insulating materials but even only that has a thermal conductivity of around 0.013 W/mK ... So you need 60mm of Aerogel if you want a U-Value of 0.2 W/m^2K. Any other type of insulation would need to be thicker to achieve 0.2 W/m^2K. Size matters as they say.
If someone out there thinks they've found a new super thin insulation (say <25mm or even <50mm) that achieves 0.2 W/m^2 then show me a test report carried out by the BRE and I'll by shares in the company
PS. Depron is great for model aircraft..
http://www.depronfoam.co.uk/html/depron_models_gallery.html