I have been using the UP 24 for a little over a month now, and I really like it. Below I'll discuss my opinions of the features.
Workout/activity featureThis gives you a way to start and stop tracking before and after a workout and then later specify what the activity was to get a better calorie burn estimate.
CalibrationThis lets you set the UP to a more exact estimate of distance by using the activity tracking feature for a walk of known distance and then telling the up what the actual distance was after you are finished. After doing this, I saw about a 20% decrease in distance so this may be important for other tall folks like myself.
Food trackingI haven't used the built-in food tracking, but the integration with My Fitness Pal is really solid. The up can update your totals in MFP and add extra exercise calories for you to know when you have more room to feast that day. Your food gets imported into the built-in food tracking so you can get tips and advice about your diet there.
RunningThis is not really a running tracker by itself, but I'm happy to report that it integrates really well with other services and (indirectly) devices. There is integration with Runkeeper, Strava, and MapMyFitness to name a few. I used to use a smartphone with Runkeeper to track my runs, but I moved to using a Garmin Forerunner 110 GPS Watch with Heart Rate Monitor which is one of the cheaper heart-rate chest strap/GPS watches I found at the time. The way I sync is by uploading from the Forerunner to MapMyFitness (through their site with the Garmin Express browser plugin). MapMyFitness syncs with the UP and feeds in my workout data so the UP knows that all the steps during a certain time were really part of this run which has a presumably more accurate calorie burn estimate with GPS/HR data than steps alone. This activity then generates calories adjustments in my food tracker app of choice (via Up Integration). In this sort of setup, it is important to connect each app to up without connecting them to each other to avoid double counts such as Map My Fitness -> UP -> My Fitness Pal and Map My Fitness -> My Fitness Pal counting the same exercise.
I do wish I had a GPS watch with bluetooth so it could automatically upload to my phone rather than having to go sit at my desktop, but that is not a failing of the UP. In fact, I notice it so much because the UP quietly syncs all the time via its low power bluetooth. When you track runs through a third-party app, you don't have to start/stop the activity timer on the up. I wear it to keep my step count accurate, but it just passively monitors and then gets updated with the activity.
SleepThe up can track your sleep pretty nicely. It will show you when you are up, in deep sleep, or in light sleep. They claim the MotionX tech allows it to be about 95% as accurate as a professional sleep study in this regard because it was calibrated against actual sleep study data. I thought this was pretty cool before I got it, but now I would say it alone may justify buying the UP 24. It makes you very aware of how much or little you are sleeping AND the quality of that sleep. Seeing a graph of your actual deep/light sleep time is infinitely more useful than just asking yourself if you feel tired or awake that day. It allows you to know when your sleep is bad so you can make some adjustments to your exercise, bedtime, eating, or caffeine.
To activate this feature, you just hold the button until the moon lights up on the band. When you wake up, just hold it down again. The Caffeine app allows you to track your caffeine consumption and can advise you on intake based on how it impacts your sleep. This app does not yet exist for Android, but it is on iPhone.
You set a goal for nightly sleep, and the app will tell you what percentage of that goal was achieved.
This feature allows the band will vibrate to wake you within a certain number of minutes of some time that you set in order to wake you up when you are at a lighter point in your sleep cycle. This supposedly leaves you feeling more refreshed. You set the time range and the wake up target time. You also set the days of the week this alarm should fire. I use this instead of a regular alarm clock now, and my waking is usually more pleasant.
This allows you to take a short nap and get woken up by vibration. To activate, you tap and then hold the button until it lights up.
Step TrackingThe step tracking seems fairly accurate, and it displays the frequency of your steps in periods throughout the day as a graph of yellow to red bars on a histogram. It shows you percentage of your goal steps and lets you know when you are on a multiday streak of meeting your goals which can be motivating.
ChargingIt uses a customized USB cable with a headphone adapter to charge. It charges very quickly, and can run for about 15 days maximum. I usually charge it one or two mornings a week while I shower.
Activity FeedThe app will give you tailored recommendations and tips based on sleep, food intake, and exercise. So far, it has pointed me to some pretty good information about running techniques, diet, and general health.
I've been quite happy with the UP, and I would recommend it.