Good service and call quality
I've been with all networks over the years, and returned to Vodafone because they had coverage in some remote areas of Scotland that I visit a lot. Although perhaps slightly more expensive than other networks, I do feel that I've bought into a quality product, with less dropped calls, faster 3G (where available), and customer service seems to be good. The coverage checker is up to date and informative. Also, I work overseas quite a lot and the included Euro data plus Vodafone passport takes a lot of the hassle and expense out of this.