So what is out there, what does it do and how can it help, hardware first, that'll get the pricey stuff out of the way, and then we'll look at software. Later there'll be a winter version looking at stuff for the pain cave.
Hardware
Bike Computers
There's a ridiculously wide range of cycling computers, they all do similar things, it comes down to personal preference and budget. There's one thing to avoid in my opinion and that's anything that does not allow a full suite of sensors to be attached to it, you might not want them now, but in the future that may change. For instance the Garmin Explore range, they look very very similar to the edge range, but are more navigation oriented, however the edge range does everything the explore does and more. They might have a slightly more attractive price, but it's not worth it.
They should all work in same way, sensors attach via a method called ANT+ or BTLE (a low energy version of BlueTooth), in theory any sensor that supports one of those can talk to a head unit that supports it. So you don't need to buy Wahoo chest strap to go with a wahoo head unit etc. Don't buy a head unit that doesn't use ANT+, it'll be proprietary and lock you into accessories.
Looking at new entrants into the field, be careful, all headunits will generally work by recording the ride on the unit and then passing it via an app or via wifi to their software servers, which might then be pickedup by other software (strava for instance). What happens if they go bust? and their servers stop (they are not free to run), your unit is a brick at this point, yes it'll probably work during the ride, but that'll be all. So be aware of new entrants, they might not be around in 12months and you might need to buy a new device. CooSpo (see HRMs below) have released a head unit, I wouldn't use this for another year at least, but would happily use their accessories.
Do you need the top of the range one? Probably not, a bigger screen might help, a bigger battery might help, i've only ever used 500 series garmins, i don't want touch screen and given the choice of paying c£200 for a 5XX or £600 for 1XXX i'll keep the money for something that will benefit me. Some will claim to be more 'aero', and honestly unless everything else is dialled in perfectly it'll make no difference, nothing you should pay more than £10 of price difference for anyway.
Be very alert for sales, the previous generation will often go on sale before the new one arrives to clear stock. Does the new one really add much? Stepping up from my 520plus to a 530 added a lot of useful info if I was training hard... would I miss it? likely not, but I would miss the responsiveness of the newer devices.
So are they useful? Yes, definitely (I'm sure some old hands will disagree), being able to see what you are doing can be useful, being able to see what you've done is definitely use, are you getting better?
Learning how to use them and personalise them is key, for instance I (and i'll readily admit that i'm not the greatest rider) have quite a detailed layout for normal riding, speed, cadence, distance HR, Power etc. about 7 fields in total I think. But when Time Trialling, I tried 4 fields and i'm now down to 2, Just HR and Power. essentially my brain turns to mush, so I need it simple. In fact it's a guage that I try and keep pointing directly up for power, and my HR. There's loads of little widgets that might help you, so have a look, again this is part of the reason to choose one system over another, the range of little assistants.
Personally I use Garmin, BUT I feel that they are missing a trick that the newer entrants have beaten them on, building new screens on the device is a pain compared to the some of the others where you can use an App. They all have widgets but looking at Garmin's selection it's poorly curated, unsearchable and of poor quality.
Cadence Sensors & Speed Sensors
One of the range of ANT+ devices, broadly there are two types, ones that need a magnet (the older style) and ones that don't. If they are an Ant+ device then they'll work with any headunit, no need to buy a garmin for a garmin and a wahoo for a wahoo. Buy what you can get, buy based on battery life, colour, whatever, but no need to match the headunit brand.
Heart Rate Straps
Again another ANT+ device, straps are better than watches, straps read the electrical signal vs bouncing a laser off your skin which I've always found to be a bit/lot off at higher levels. Again any brand will work with any headunit, I had a garmin strap, moved to a CooSpo when my garmin failed, had it for 6 years now maybe? Interestingly the garmin would regularly give me a false reading of 240bpm between waltham and stonesby, downhill, the CooSpo never has. There'll be other up and coming chinese brands that will offer cheaper versions until they get big.
Some sports bras have the connections for the sensor, I'd expect that they are standard spacings, however a normal strap may well still work.
There are typically two ways that they are powered a CR2032 battery, or rechargable. The batteries last for maybe a year or more. Not worth the rechargeable cost in my opinion.
There are 'better' HRMs that track more than just BPM, I have no opinion other than unless you are a pro with a medical team behind you, or have a specific condition, then would you know what to do with the data beyond BPM?
Power Meters
The holy grail of sensors, again ANT+ (there might have been units in the past that needed a dedicated head unit). Broadly 3 options:
Hub Mounted, replace your rear wheel hub and it'll detect torque and therefore power applied to the wheel. Needs a special hub, likely a new set of spokes and obviously a wheel rebuild. Does detect power from both legs, but only as a total.
Crank/Spider mounted, a device attached to crank and the spider on the drive side detects the tiny twisting and bending when pedalling, typically non-drive only or both sides. If one sided then it estimates the total power assuming a 50:50 split. Will also double as a cadence sensor, so you might save a (tiny) bit. Make sure that your crank fits, for shimano at least swapping cranks is easy, so i'm using one crank and swapping winter to summer bike and back. You can fit the pods to existing cranks and spiders, but it's typically something that you send them away for.
Pedal mounted, a newer entrant to the field, only two companies worth talking about, Favero or Garmin, typically they have the electronics on the pedal spindle, or in a pod. Favero do have a spindle only option for higher end (ultegra or better) SPD-SL pedals where you replace the spindle. There might be concerns about sealing these as there are moving parts that need a decent seal against water, which is not easy and can be unreliable, crank based are static. There may be concerns about the 'Q' dimension, this is essentially the distance between the pedals, which can increase with pedal mounted devices. All 4 of my bikes have a different Q dimensions, there seems to be no difference for tall or short people and yet we'd have different hip widths, personally I don't see an issue, measure your own bikes and see if you see differences.
There has been news from Eurobike, that there are new entrants to the field coming out for crank based meters, so there might be some competition on price in the near future.
One company recently failed, may have been brought out, i'd happily buy their product if it's going for a decent price, all the communication is to the headunit, it's only warranty you'd have to watch for and they are pretty robust.