I have the Flex version, which is older, but it does everything I need! You have to synch it with an app on your phone to see your exact step count, but for me, that's a small price to pay. I don't feel the need to upgrade to one with a watch (mostly because I already wear a beautiful Kate Spade watch and also never take it off) or with stair-tracking functions, but the people I know with the more high-tech Fitbits love them dearly as well.
I'm such a competitive person and my Fitbit fuels that competition into being active. You can connect with friends and challenge them with steps, and it's so much fun. I totally get wrapped up in beating them that I'll go to the gym and walk/run for hours just to get ahead!
My absolute favorite part about my Fitbit though is the sleep tracking function. I have always thought sleep trackers are so fascinating. I've used several apps on my phone before, but I just really don't enjoy sleeping with my phone beneath my pillow or on my bed at all. Plus, on the off chance it falls off the bed during the night, none of your sleep is tracked! I barely notice my Fitbit anymore so falling asleep with it on is so easy.
Now that Fitbits are such a big trend, more and more designers are beginning to make "stylish" bands so you can still wear it, but have it at least a little more inconspicuous. Tory Burch is partnered with Fitbit, so they're a bit more pricey, but you can find more on Etsy and other shops.
I really am obsessed with my Fitbit. It has definitely encouraged me to be more active and to get up and move!