I had a set on a 2nd hand Kona Hei Hei 29er.
Lighter than alloy, never had an issue with them, you can get guards for covering the tips plus heli tape to avoid rub from your shoes.
Competitive racer, worth the extra. Me, nowt wrong with normal cranks.
I'd consider getting them IF you need to get cranks and the price difference is bareable. Wouldn't waste money on upgrading just to move to carbon fibre.
Got some on my 29er, came on the bike as new.
Can't say I can tell the difference, but small weight savings here and there add up.
Get scraped quite easily from rock strikes, but cosmetic only - got myself some crank boots protect against further scrapes.
Don't think I'm good enough to warrant paying full price for them, but wouldn't hesitate on a good discount.