Is it just a better road feel or does carbon provide some other benefit? Or is it just about the quality of the wheel bearings that determines how fast a wheel will be?
I’d say that the biggest benefits of carbon over alloy is weight and the ability to produce a variety of shapes.
You are correct that hubs and bearings also have a big effect on speed also.
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Way better.
Crabs on fire.
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there is simply more possible with carbon, you can stretch the limits further with “cheap/light/strong pick 2”
alu is always cheap, and can be either strong and heavy or light an weak.
You can go further on those limits with carbon, due to the manufacturing options (different fibers, combinations, layup, quality control etc. Resulting in for example a really light and pretty strong expensive rim.
The most important thing is that carbon looks better.
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What I can never get my head around is are they proportionally better in terms of monetary value?
I guess its rather subjective but £/W or £/improvement is, imo, ridiculous.
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Better - yes, Worth the extra dosh - not so sure. But if carbon wheels get you excited to ride your bike, then in the long run it gets you way faster.
I moved from a 32 spoke 2kg gravel wheel set with gator skins to a tubeless 24 spoke 45mm deep carbon wheel set up tubeless - I probably got about 1kmh faster, but my bike looked hella better
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Yes and in the age of disc brakes and direct to consumer sales, the internal business case is a lot easier. The £ delta between some of my alloy sets and carbon is not that big. A carbon set with perfectly OK hubs is close to an alloy set with shinier hubs.
On the mountain bike I find that carbon tracks like it’s on rails around corners. Aluminum always has a vague wandering that I’m assuming is due to the flex. Not sure if this would translate to road riding as well.
The bearings and hubs make a huge difference!
Remember, there’s a big difference between poor carbon wheels vs excellent alloy wheels.
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Hot take: on a hardtail, I’d rather have a good set of aluminum wheels than carbon. On the constantly bumpy trails in my part of North Carolina, the extreme stiffness of carbon is not comfortable and I find myself out of the saddle more and fatiguing faster. That extra bit of compliance in aluminum wheels makes a real difference (or at least I’ve convinced myself that it does). Love carbon on a full sus though.
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I’ve had a google around without much success, interested in whether this can be quantified.
The only numbers I could find are across the entire drive train and from a company who sell ceramic bearings.
With CeramicSpeed products in your hubs, pulley wheels and bottom bracket, tests show that you’ll save between 6–9 watts , compared to using standard bearings
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I use good quality steel bearings and I’m happy with how smooth my hubs run. Don’t get me wrong, I like the ‘bling’ part of having ceramic bearings but I’m not sure if they’re worth the money. It seems the only people that ‘big them up’ is the company selling them.
I do run the ceramic speed jockey wheels and they do spin better than Shimano, but the real reason for buying them was because they were red.
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They aren’t…ceramic bearings are almost worthless in a bike application.
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To me the primary benefit is aerodynamic and lighter for the same shape in aluminum especially as you go deeper.
IMO, ride quality on my 50mm rims is worse than my old 23mm deep aluminum rims.
Definitely not. Honestly if one is a slower rider and isn’t competing or trying to save 15 watts so they can hang on a group ride then carbon rims have little relative value.
I personally don’ t understand people buying expensive carbon gravel rims only to put wide knobby tires on them and defeat all the aerodynamics.
I’m not really into the mountain bike scene but I think there is a case for a really strong carbon rim that can be abused and that won’t dent or break. Don’t quote me on that though.
If you use the right, softer pads you’ll have almost zero wear on your brake tracks unless you ride in the wet. I highly recommend Campagnolo red pads. They were a game changer for me.
I would assume this is more due to how the different wheels were built - spoke geometry, spoke tension.
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Yep same - in my experience carbon tracks wear far slower than aluminium - but that could either just be down to my particular wheels (campy bora wto60) or the fact I don’t like riding my nice bike in the rain 
Are carbon frames better?
Are carbon bars better?
Are carbon seatposts better?
I only have one bike on aluminum wheels right now, which is my budget gravel/commuter bike. Only bike I would want to consider running aluminum wheels on is a downhill bike.
Can’t remember the guy’s name (Josh something) who I think works for Silca and was recently interviewed by Dylan Johnson talks about how ceramic bearings are only faster when they are constantly maintained. If you don’t maintain them (much like a pro tour team would), then they end up being worse.
I currently have 27000 miles on my rim brake wheels, and I live in a very hilly area (my chill century ride Sunday was 7000’ of climbing). Still a ton of life left in them.
Not sure if my elevation gain means much as I don’t use the brakes much descending 
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The main advantage of carbon wheels is you can make deep aero wheels (like 50mm+) much lighter. An alloy wheel at that depth would be a boat anchor. The weight advantages at shallower depths can still be there, but aren’t as dramatic, and I personally don’t see much point in shallow carbon wheels in most cases (for example something like a Zipp 202 isn’t aero or light).
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I still train on alloy rims, but I’m a retro grouch or sorts, still using rim brakes.
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Thanks for all the replies. Good to read everyone’s opinions.