I wouldn't be too quick to associate myself with SD. They are very much English based and don't even have any Board representation from Scotland.
However, they do have links with the SFA/SPFL and receive funding from them as well as the English FA. That means that they are not a truly independent Fans organisation, and are now effectively a tame in-house puppet organisation for both the SFA and FA. (sounds a bit familiar)
I believe that Paul Goodwin, who was their Scottish based rep. until two or three years ago, was forced out at the behest of Petrie and co., because SD Scotland was becoming a bit too vocal for their liking, despite their work in helping Hearts, Dunfermline and other clubs in recent years.
I know that an independent organisation called Fans Supporters Federation (FSF) was set up in England and now has 500K members, which means that the FA and Westminster government agencies can't ignore them. Similarly, Paul Goodwin has set up a rival supporters group in Scotland called the Scottish Football Supporters Association (SFSA) with the intention of obtaining similar leverage with Scottish clubs, the SFA and the government.
I think SD, FSF and the SFSA can all provide good advice and support, but personally I'd prefer to be associated with a truly independent fans group.