I wouldn't be bothering just for myself, but at some point I will be initiating a DNA study as part of a one name study. I think that is really where the huge benefits can come in. I think I'd encourage anyone interested in the whole DNA thing to see if there is a surname society for their name, or a one-name study, and see what they are doing first. The benefits there are that you can make a lot more headway in linking larger groups of people together, or proving some of those connections from quite some generations back that you had to just guess at before.
I really don't see the benefits of just getting tested as an individual and finding someone else with a common ancestor 6 generations back, if you are just researching your own immediate family. If you are looking at cost as well, most of the surname societies or one-name studies can get test kits cheaper one way or another - either by funding them from donations from members of the group or buying in bulk, so thats another reason to go with an already recognised study group.
Having said all that, are there any male Sennett's out there (from any line) who would be interested in getting their DNA recorded? As the administrator of the Sennett one-name study, I have a hunch that DNA testing will find that several of the (at the moment) separate lines of Sennetts in the UK will end up having the same common ancestor in Ireland.