I've done one, 23 and Me, and I'm about to do another one.
I think there are positives and negatives. I enjoyed learning about my DNA, which has helped me somewhat with a family mystery I was trying to solve.
On the basis of my experience, I would say that the DNA test alone won't be much good for helping your Mum learn about her family tree in a specific sense. You really need to be prepared to do the research as well that goes with it - otherwise all you really have is a list of living relatives around the world, and while that's interesting and that won't be that helpful if you haven't got a written tree to back it up and help you work out how those people are linked.
It's also only as good as the number of other people who have submitted their DNA. So for example, I have a few matches that seem to be drawn out of my Irish family because lots of Americans with Irish DNA are interested in looking at their family roots. Another branch of my family is entirely without matches as far as I can tell, probably because that side of the family just aren't interested. (I know we are related because it's my maternal line).
I would suggest getting the Ancestry one, because you can get an Ancestry Tree account along with it that will tie in to your results and help you to do the research.
People are correct that you run the risk of turning up a family secret, such as a half sibling with these tests. However that's a risk even when you are exploring your family tree on paper. For example, I recently found out that someone who we thought was a relatives Aunt was actually their half-sister. Which is bloody awkward for me to deal with - that was all on paper and had nothing to do with my DNA test.
I'm still glad I did mine.