Like all cross training, on the CV side, its as effective as your HRM is telling you. I.e your heart doesn't differentiate between HR increase from one activity and another. However, cross-training is only effective in training of the primary sports muscle & skills if there is a large number of shared movements. i.e. Rotary arm cycles down the gym will not develop the skills associated with running, or the muscle strength/cappilarization.
Having said that, football does involve lots of running! The only bit of the training that would have little 'crossover' would be the kicking action, but having said that, I used to count football as cross-training, and Its a dang site more fun than the treadmill IMHO :)