I've gone the other route and done 1 at a time on my '04 F150. To date, 5 of them have been replaced between 85,000 miles and 182,000 miles. At nearly $50 a crack, a full swap would've been $400. Every failure has been the same - a series of small misfires shows up under light acceleration between 40 -and 50 mph. The shuddering can linger for months before it will ever set a code, but using torque pro I can pull the misfire counts. Since I am now a beater mode, happy with my choice to piecemeal it...
(My '07 Explorer uses the same coils, and to date has had one replaced at 115,000 miles).