Thank you @thenewaveragebear1983, you are so right about the primacy of personal experience over theory, especially when "theory" incorporates so many conflicting or divergent ideas. Can't help feeling that nutritional medicine is currently at the same stage that surgery was in, say, the 17th century, before we had an understanding of germ theory, or anesthetics, let alone antibiotics. We have so far to go. The whole "fat is bad for you" debacle is proof of that.
I do have a basic fitbit, and in the past I have had an assortment of Suunto and Polar devices, back into the mists of time. (If I remember correctly the first one had a little hatch on the side, into which you inserted a tiny lump of coal.)
Anyway, the fitbit has been a mixed experience for me, partly due to charging problems. I no longer do cardio like running or treadmills, but I did use the fitbit for some activities, mainly long walks in hilly terrain, where it functioned well most of the time.
More generally, I have found that when you're doing work that involves carrying heavy things, anything attached to your wrist tends to get damaged because those items (e.g. rounds of wood or logs) push or scrape against it when you pick them up. Also any band tight enough to give a good contact tends to be uncomfortable / get damaged when putting a lot of force through wrists and forearms, such as when training with weights, kettlebells in particular, or using an axe to split wood. But YMMV.
Not too sure about circuit training. If it includes reasonable weights, maybe you get the "afterburn" effect posited for resistance training by some researchers, so might be better than vanilla cardio? But I too would think that the 455 cals is optimistic. Don't know why these devices do it - equivalent to vanity sizing, but for exercise?
One fitbit device I use daily and that has worked really well is my Aria scale. I know it's not cheap, but I heartily recommend it. Like another poster to the thread, I weigh myself daily. It does fluctuate, but it keeps me honest, and the data goes straight to the cloud, just like it does with a fitbit HRM. Means you can gaze at charts later in despair/elation, as applicable.