I don't actually do Fly but I can see the appeal. My house is fairly clean and tidy these days (DDs grown up, that's why!) and I have a routine which includes hoovering carpeted bits, sweeping wood floors, bathrooms etc. But however much better it all looks when those things are done there are always things I spot along the way which also need doing - inside cupboards, dusting bookshelves in out-of-the-way places, skirting boards etc and there are so many of these I shy away from knowing where to start and seldom do any at all.
Someone else deciding FOR me and giving me one small extra task a day to do which I wouldn't normally would be an excellent motivator for me.
For example, I went to the site once and found the day's task was 'declutter the kitchen worktops for 15 mins' - you had to set an alarm and STOP when the 15 mins was up - and I thought how psychologically cunning that was - someone looking around and panicking at the chaos they see and not knowing where to start would find it easy to do a 15 min task knowing that they not only could, but must stop at the end of the allotted time.
I'm not obsessive about housework and I'm sure most Fly ladies aren't either, but I do enjoy my house more when it's clean and tidy - I can't, for example, relax until the kitchen's clean and tidy again after a cooking session - not because I neurotically hate the look of it - but because I know it's waiting there for me to do!