To get fitbit to count spinning I wear it on the leg of my bike shorts. I think it might undercount "steps" this way since you are only getting one side counted, but it is more accurate than wearing the device at your waist. I know this because one day DH forgot his fitbit at home and I wore both devices, one on my leg and one at my waist, and the leg one counted probably 50% more "steps".
Fitbit does a much better job at counting calories of anything with steps (it is basically a fancy pedometer). So weight training, for example, it doesn't count well. You can override the time spent weight training on the website, but since I am not PERFECT on logging my food, I don't worry about the weight training days and just try to still get in my 5 miles. I believe it uses some sort of algorithm based on age, height, speed of acquiring steps, etc. to calculate your calorie burn.
I love my fitbit and would replace it if anything happened to it.
Instead of using the fitbit website to log food, I use myfitnesspal on my iPhone and it syncs with fitbit which is totally cool. I used to log exercise manually on MFP, but now the fitbit sends over the calorie burn info and I use that to figure out how many calories I can eat each day. DH uses the fitbit website to log food and I don't think it is as slick as MFP, also, not as good of a food database and he is constantly asking me how many calories I logged for dinner.
Between DH and me, we should get some sort of royalty from fitbit because at least 8 people now have fitbits on our recommendation! However, I will say that it is essentially a toy and I was able to lose the 15 lbs I lost mostly due to food logging. So I do consider the food log to be more critical to one's success as opposed to the fitbit, but I enjoy the fitbit more than the food logging!
