I am using Gina Ford's routines, however I totally agree with the above posters that it will depend on both the baby and the mother/ primary carer if it will suit you or not, I can well imagine that it might not work well for everyone.
I fully intended to demand feed, at least for the first 3 months, but we were given the book by a friend who uses it and started using the routines at 2 1/2 weeks old for the following reasons:
1. Our baby was very 'sleepy' to start off with. We were having to wake him up anyway while demand feeding to feed as he was sleeping longer than the 4 hours during the day and 6 hours during the night, that is reccommened as the maximum for new borns.
2. He was continuously sleeping except for the times we woke him, as mentioned above, for most of the day / night, but then having all of his awake time in one stretch, up to 8 hours at a time, even with regular feeds during that time, and as you can imagine howling that whole time, which was exhausting us all.
The benefits of using the routine for us is that he has developed very good sleep associations. He will self settle in his cot for naps and evening sleeps and only wakes up once overnight for a feed. At 6 weeks, he is already looking like getting close to dropping that overnight feed.
It has trained him to spread his sleep out properly, so that he is not fretting for long periods at a time as he was before.
For myself, I am very structured usually in my working / personal life, running to a strict schedule of meetings etc, so a routine is actually a relief for me.
For example I know with certainty now that he will definitely be asleep between the hours of 11:30 am and 2 pm, so I can plan my day to run errands or make appointments to get a hair cut etc, during that time while he is asleep at home.
With regards to outings during the day WITH him, I just relax the schedule to do that. When we are at home, we stick fairly strictly to it, so that he establishes the routine, but he is fine if we vary it a bit on the days we are out. I don't ever let his routine restrict mine at all.
So say for the scenario of going out to lunch with friends mentioned above, I'll just take him with me asleep in his pram to lunch and if I'm still out for his feeding time, do that while I'm out as well. In my case I am breast feeding, so I find it really easy to just feed him where ever I am.
Actually, again, in a way I find the routine EASIER to manage outings with him, as both he and I know what and when to expect. With demand feeding you do still need to handle your baby feeding and sleeping while you are out, so I don't see that it is more inconvenient.
As I mentioned above, I am only outlining why it works for us, I am not trying to advocate that it is the best way to do things. Give it a go if you think it might suit you, but keep in mind, especially with an older baby, I imagine, that it will take a week or so to establish, the routine does not just magically 'click' into place.
Good luck.