From what I gathered earlier this spring, no carrier is totally reliable there. I don't remember all of the options but I do remember that if I decide I need to bring a cellphone to StJ, that I'll go with Cingular.
We have Cingular and had no problems really except for some spotty service. Our friends had Verizon and had so much trouble finding a signal that they gave up and used our phones.
Well, in November I had problems with Cingular. I'll know tomorrow if I get coverage in Coral Bay. I also have a Cingular wireless broadband card, so I'm hoping to get some signal on Ajax Mountain.
We've used Cingular the last 3 times we have been on St John. No problems, in fact, I got better reception there than in the school I work at here at home!