I agree with Nick that autosomal DNA is generally not very helpful for filling in one's family tree. Unless you get lucky. It is probably a little more helpful for Americans just because more have tested than in the UK---but that may change with time. Its really a matter of luck and coverage. My parents are both deceased but my offspring's tests reveal they random nature of DNA. When I compare someone I match with with my children sometimes they inherit a segment intact, sometimes not at all and sometimes one gets part and the other nothing. I also have results for a first cousin and a second cousin---that's where it gets more interesting as you can start to see where segments come from.
This works for me because I have a very diverse background and some of it was previously unknown. For instance I have a match with a man from Mexico and it happens to be on a segment where we both "paint" Native American (Indian). For those who have done autosomal testing the most useful tools I have found are on GEDMATCH.com You upload your data and then you can use many tools for discerning geographic ancestry etc.
If you are 100% Scottish then perhaps it won't tell you as much---but then again you may get a few surprises too as people have been mixing it up for thousands of years. Some Scots will trace back to Vikings and some may have ancient Celtic and some more recent Roman Celtic etc. and every now and then there will turn up African or Portuguese or Asian in someplace where it is least expected and that then turns into a history lesson on the Spread of various empires and the sea-faring nations etc.
So whereas the Y-DNA or mtDNA will tell you about your respective paternal and maternal lines---your autosomal DNA will tell you something about all the ancestors from whom you still retain DNA. And here again it is a numbers game. You will absolutely have DNA passed down from your great-great-great grandparents but as you move further back there will be some from whom you retain large segments and others from whom you inherit nothing.
And it is important to remember that the SNPs these companies are testing for are the ones that tend to show differences---the rest of it is 99.9% identical. So they are looking at 700,000-900,000 SNPs. It is not clear that full genome processing will provide much more relevant data---time will tell.
The new Ancestry.com beta autosomal test may be the key to real austosomal testing usefulness. If enough people test and it is tied to so many family trees and so many people who are truly interested in genealogy it may make what now seems impossible possible. I remember when you had to search census records by hand---there were no indexes. So in some ways it is just a waiting game.
