Yup. I use an older model Wahoo sensor set with my iPhone 4s on my old steel road bike for indoor trainer. There are still plenty of unsold new/old stock Wahoo Fitness Bike Pack kits for iPhone 3 to 4 models that include:

• ANT+ adapter in a weather resistant case
• Speed sensor (for rear wheel where it's most useful if we use our bikes on rollers or Cycleops type trainers)
• Cadence sensor

Costs a whopping $15. Retail was $150 when this kit was new in 2012. Still works fine. But it's heavy and bulky so I only use it on the indoor trainer.

I also have the Wahoo Tickr heart rate monitor. I wore it constantly for a month but now only wear it on workouts. It pretty much confirmed what I already knew. However in the couple of months I've worn it I have noticed a slight uptick in my maximum ticker rate, so it wasn't a complete waste of money. But pricey at $50. The older Wahoo HR monitor would have been a better buy for my old iPhone.

I don't bother with this stuff on outdoor rides for various reasons. I just use the Wahoo Fitness and Strava apps. The phone is in my pocket, not on the handlebar:

• My cadence is like clockwork, 90 rpm once I'm warmed up. I don't need a device to remind me.
• Speed? Meh, don't really care. My training routes are on windy rolling prairie. The wind can affect my speed differently every day. I just go by perceived effort. And in group rides I ride the group pace, so I don't need to know my speed. Can I keep up with the group or my riding buddy? I'm fast enough.

I do have a good cheap bike computer, wireless, no interaction with my smartphones or GPS. Cost about $12. Weighs next to nothing. I might put it on my carbon bike. Mostly it's helpful to confirm my average speed, distance, etc., in case there's a GPS sync glitch with my phone and/or fitness apps. When I used the bike computer on the handlebar it usually matched my GPS data, within a few nths -- close enough for my purposes.

If I got serious about training for time trials I might consider a power meter, but it's difficult to justify the cost. My bikes don't even cost more than $200, so I can't justify $500 for a power meter. I already know I'm on the low performance rung among guys my age. If I improve to where I can keep up with other serious amateur 60somethings, then I might be ready to finesse my training. But I'd rather rent one or work out in a gym with a cycling coach.