DH has a Fitbit Flex. I have a Fitbit One and my Fitbit Force is on the way to me right now.
I briefly use a WW Active LInk (which basically works with the WW system) which is made by Philips.
I do have heart rate monitors which I use when working out. The Fitbit I wear all the time. I looked into other monitors including the Jawbone but the research I read seemed to find the Fitbit the most accurate. FWIW, since having my One (and DH having his Flex) we have both checked the accuracy and both are very accurate.
Basically with either the Flex or the One or the new Force, you wear the device all the time (DH wears the Flex in the shower but the One can't go in the shower and I think it is not advisable for the Force although that is not entirely clear). It measures your activity. It measures it most clearly when you are doing something like walking where you are moving your legs. It does show activity when doing other things (lifting weights, for example) but you get the most accurate information by going in and "naming" the activity later. For example, I will name weightlifting or rowing on my Concept2 to get more accurate information.
Where this has been an eye opener to me is in seeing exactly how many (or how few) calories I burn in my daily life. I can look at the log and see throughout the day how what I do burns calories (or doesn't). In my case I found that I was burning much fewer calories than I would have expected. It is, however, quite motivating to do something more active and then see all the calories that I did burn.
In my case, I am interested in weight loss so I sync the Fitbit with myfitnesspal where I record what I eat each day. That way I can see clearly how many calories I ate versus how many I burned (yes I have found that this is actually quite accurate as shown by my weekly weigh in at WW).
The other thing is that it is motivating to set a step goal and see how close I am to that goal. There certainly have been days where I was more active because I wanted to reach the next goal.
With Fitbit you only have to pay for the monitor and there is no ongoing charge. You can do a premium membership (maybe $50 a year - I forget) where you can see how your results compare to others (as a group) and can set a training goal for a certain amount of activity. This is a nice to have if you like to quantify things, but it is very optional (I paid for it, DH didn't).
You can look at an activity and see how many calories you burned during it, how far you walked (or ran), how many steps, and your pace.
You can sync to your computer by getting the device near it (there is a USB dongle). You have to plug in the device to charge it. If you have an iPhone or the newest Android (forget the model) you can sync to your phone (or your iPad).
Until my Force gets here, I have the One. I keep it in my pocket. It is nice because you can scroll through a display on it and see your total steps, how many calories burned, distance, time of day. You can wear a band and night and it will track your sleep. A positive is that it is unobtrusive, being in my pocket. On the other hand, I always have to wear something with a pocket and it is easy to forget about it. In addition to everything I mentioned above it will also tell you how many stairs you climbed. Also the band at night is sort of annoying to wear.
DH has the Flex which you wear on the wrist. Unlike the One, it does not measure stairs climbed. However, he can wear his in the shower. The reason I didn't get the flex is that it doesn't have a display giving all your numbers. DH doesn't really care - he just looks it on the computer. For me, if I'm out taking a long walk I like to be able to look and see how far I've walked and my number of steps. I also like seeing how many calories I've burned so far. I like that you can wear it and forget about it and don't have to worry about having in your pocket. Also I think it is more comfortable for measuring sleep. You can set an alarm (from your computer) and it will vibrate to wake you in the morning.
The Force just came out (I think Best Buy has it in stock). I have one being shipping to me right now. The Force is like a Flex that cures everything I didn't like about the Flex. It has a display which you can rotate through to see the stats. Also, it does not have to be removed from its band to charge (on the other hand, you can change bands with the Flex and can't do that with the Force). It measures altitude so will count stairs. Another cool thing is that with my iPhone 5S I will be able to have calls to my phone buzz my Force with the number calling being shown on the Force. (I think this is optional to use if you want. This requires a software update and isn't active yet). THe only negatives of the Force versus the Flex is that the Force is a little larger (didn't bother me) and is not as water resistant as the Flex (I would never wear it in the shower either way so that didn't bother me).