I've tried all sorts of stuff over my time with diesel motors. I ran a 406 2.2 HDi between 2005 and 2009), and am now using a Passat 2.0 TDi 170.
In general I think it's better to make gentle use of additives to maintain cleanliness than to dose up hard when you have a problem. As such I cannot vouch for any additive in particular but I do think that decent fuel is a starting point for any car.
What I can say is that I've not yet had an emissions problem with any car of mine. I don't like the word never, nor the word ever, they imply all of eternity: past, present and future.
So I can conclude that additives possibly do work but I have no hard evidence to support this. I can conclude that for me they have certainly done no damage. Note however that I NEVER use engine oil additives.
The interesting one for me is using part synthetic two stroke oil (JASO FC low ash) mixed in with diesel fuel. It's widely discussed on the 'net and I have been adding small doses of 2T oil to my diesel since mid summer last year, so for about 15,000 miles. By low I mean 1:200 or even lower. This certainly quietens down the noisy PD injectors in my Passat and the exhaust now emits no soot at all (this is a non DPF car).
I run it on BP Ultimate most of the time with the occasional half tank of standard diesel just for the hell of it.
I don't use supermarket fuel in any car, it's akin to adding sand to a bechamel sauce in my opinion, less than ideal