I have a MS Band 2. It's one of few out there that has GPS built into it. You don't need to bring your phone on a run or bike ride. It tracks heart rate. It seems to be very accurate with regard to calorie burn. The MS Health dashboard (browser app) has everything you need with summaries and details of all of your activities. Being able to see all of the stats laid out in front of you and your numbers over time can be very motivating. I'm always trying to beat my fastest time or greatest distance.
The only things I'd like to see if they ever came out with a Band 3 is the ability to store music on it and Bluetooth to some fitness headphones. Further, I'd like it to be fully water-resistant and incorporate swimming workouts/activities. Also, I've found through almost five months of wearing it, the band holds up well, but the contacts you connect the charger to are prone to corrosion from your sweat. Even if you wipe the thing down or clean it really well where the charging contacts are, it still develops corrosion under the seal by the contacts. This results in the contacts not always matching up with the charger. This became a problem resulting in my returning it. I should have the replacement shortly. But you'd expect something called a 'fitness' band which touts water resistance to be able to deal with sweat five days a week. That issue needs to be corrected for sure. If they make the next device water-proof rather than water-resistant, this will probably be a non-issue.
I think it's pretty cool device, but they should add the improvements and corrections I've suggested to the next iteration.