Here is a link to a six-month clinical comparison of the efficacy of the Sonicare and the Braun Oral-B electric toothbrushes on improving periodontal health in adult periodontitis patients.
http://www.ncbi.nlm.nih.gov/pubmed/9487838
To summarize: "Overall, this study demonstrates that long-term use of these two electric toothbrushes improves periodontal health in adult periodontitis patients, and that the Sonicare brush is superior to the Braun brush in reducing gingival inflammation and probing depth. Moreover, 6 months' use of Sonicare led to actual improvement in probing attachment levels of periodontal pockets."
Our family bought sonicare brushes years ago and still have them. We just get new heads every six months. As for how to use it for the whole family, you can buy a unit for each person or you can buy multiple color banded heads and swap the heads out onto the main unit to share between people. Since our family tends to all brush at the same time, we each have our own units but we all share a charging station.