I have one of Fixitsan's dongles.
it performs 2 tasks.
1:- stop the EGR error code from showing on the dash, it doesn't disable the EGR, you need a blanking plate for that. If your EGR is working properly then fitting the dongle will make no difference, If your EGR is stuck shut this will stop the error code from showing, If your EGR is stuck open I dont know what effect this will have, but you should be able to tell anyway because you'll be struggling to go anywhere fast!
2:- it regularly resets the DPF clogging timer, so if you have a bypass pipe or have drilled the DPF the ECU will constantly think it's a long time till the next regen. end result is that the regens don't happen. It doesn't do anything more fancy than reset the timer whenever it reaches a preset value. (it doesn;t need to do anything more fancy than that, it's a wonderfully simple solution!)
The dongle is not and never was intended as a performance enhancement, or an alternative to a remap. fitting one means you can blank your EGR and/or remove/drill your DPF without the dashboard errors from showing and without worrying about blocked DPF's faulty EGR valves and the reduced economy that brings.
Unfortunatly some members on here didn't understand that and put enough pressure on Fixitsan that he felt obliged to leave the forum. A sad loss as he has a wealth of technical expertise and was prepared to invest his own time and money into helping others. If he'd done it as a registered business he'd have been welcomed and rewarded, but as a private individual he was hounded and harrased.
He's a nice guy.
But blocking the EGR valve is better for the engine long term as it stops it recirculating soot and carbon deposits which is a big added bonus.
Drilling the DPF has had a big effect on the performance of mine, this maybe due to it being clogged, but not much soot came out. Removing the caps makes the exhaust flow easier even with the cats still in place so brings a performance gain.
Drilling the DPF also immediately bought an increase in MPG which was very noticeable and continues to be.
These are just my personal experiences although others have stated the same.
Finally if you have a problem with your EGR valve or DPF and aren't interested in a remap then buying a dongle, blanking the EGR valve and/or drilling the DPF costs under £20 and a little time, rather than the cost of new parts and a garage bill.
As for Fixitsan going, this forum is alot quieter and less well informed since he went. The reasons why he was pushed make me too angry to think about
so I wont.