I used a FitBit One for many years until I washed it recently and it died. Honestly, I never really looked at the data unless I expected it to be anomalous, even though I religiously carried it with me at all times. I bike 8-10 hours a week, so a few steps aren't really making a health difference for me.
The big thing that annoyed me about FitBit is that Apple has a reasonably useful thing for collecting all your health information on your iPhone, but FitBit refuses to participate (no doubt because it would cost software engineering time and reduce the number of "premium" subscribers).
All-day heart-rate tracking would be interesting, but I already wear a watch and I'm not going to wear a second one. (And I doubt you'll talk me out of wearing my Speedmaster. Maybe Patek makes a fitness-tracking watch :P)
Everything sleep-tracking related has terrible reviews, from my research. I would love to know exactly how much time I spend in bed, but nothing will collect the data for me without buggy apps and overcomplicated made-up analysis and whatnot ("you spend 3.7 minutes in REM" uh huh.)
I'm strongly considering just putting a pressure sensor under one of my bed's legs, connecting it to some WiFi-enabled resistance reader, and using that. It's surprising to me that nobody makes a product in this space that does one thing and does it well. I am also surprised that people like wearing things to bed. Like I'll remember to do that.
For biking, I think I already own every gadget there is. I've seen that there are now muscle oxygenation sensors (presumably for measuring when you're riding at lactate threshold), but the reviews say the devices basically just return random numbers and you might as well use a power meter and do an FTP test every couple months (which is good, but some days I feel like I can ride at my FTP forever, so I wonder how accurate these 20 minute tests are).
All in all it seems like people are interested in collecting data about themselves, so companies are racing to build devices to fill the gap with random numbers :P