I've always thought of FitBit as like what Apple did with the original iPod.
Until recent models that added complexity, it's had a narrow focus, been stylish, and been very simple to use.
When the Apple Watch came out, it was the antithesis of all of that (well you can argue about style). Even the original iPhone didn't launch with an App Store, but the original Apple Watch did.
I like my FitBit over an Apple Watch mainly because of the FitBit app, which also draws a parallel to the original iPod which used iTunes as its hub. If I lose a device or want to check my FitBit data anywhere anyplace, I just log onto fitbit.com or re-download the app, and all of my data is right there. On the other hand, the Apple Health app is sprawling and it's stuck on your one single phone (I think newer OS versions maybe sync it over iCloud?).
The FitBit isn't perfect at pulse, but for me my use is mainly sleep tracking and it's eerily good at that.
I really hope they keep some of the "narrow" original iPod-like models around.
I know the trend has been for all devices to be all-purpose computers, so that may not be in the long-term direction of FitBit now. Still I'm not sure why Google would buy them if it didn't see that appeal, as I don't think that FitBit's "smarter" bands are necessarily what they are most known for.
If I had not known what the future was going to hold post original iPod and you told me in the future there would be one thin band that would track your steps, heart rate, and sleep (FitBit), and one that had a sizeable color display that could run arbitrary applications and used a couple of buttons and loaded apps off of a connected cell phone, I would have guessed the first device was made by Apple and the second made by Microsoft. I wouldn't have thought of Google being involved at all. But here we are. To me the FitBit is still the most Apple-like product in this category despite it not at all being an Apple product.