I think some of it they state themselves somewhere.
Leigh's you'd have to see where she says… she has a lot of clients and they use them (and the bodybugg before it) and so I think at least partly is based on that.
Remember that in the end, everything is using estimates and formulas… and no matter how careful you are about your intake… those calorie amounts are estimates from sampling and therefore also subject to margin of error (assuming you're getting the right info in the first place, even). So at the end of the day… it's all about the individual and how you observe your trend is relative to what your "illusion" of your burn and consumption is. So, it is certainly useful for that. You can learn what you "burn" and track that with what you "consume" and figure out what difference between those 2 numbers you need for the results you want… precisely accurate or not. ya know?
If you get it you'll likely find it useful and a big help… but it remains to be seen how closely the number it gives you matches "truth." But, it's variance will be stable and not all over the place, so that's helpful at least.
I don't have one and while having contemplated it a few times (or more) at the end of the day, I find it really doesn't matter and there are more important things on my list to spend my money on. I basically do what it does… guess based on some numbers and figure out what I need to do based on the results I get. It works for me.
Which means I haven't studied up too much on it, just remembering based on previous posts and things I've read and whatnot… so I can't think of any link in particular other than the thread you've already seen and a general "Leigh's site" direction when it comes to documentation. (LeighPeele.com or flzine.com would be those sites)
Sorry can't help more… but from what I've seen, most people find it pretty accurate, it's only a few people that seem to have big disparities going on… at least around here. You may want to check out one used, perhaps less investment.