Dura Ace 9000 c24 or Fulcrum Racing Zero

+1 for the Wheelsmiths, although I have the Race23s

I can't compare as I only have C24s, but I can certainly vouch for the improvement in comfort. I went from Ksyriums to the C24s and initially stopped a couple of times to check the tyres were still fully up to pressure!

I don't know about longevity as I keep them for best, wearing GP4000S. Aksiums with Gatorskins do training duty and the difference in feel between the two sets is like riding a different bike.

A friend of mine has 7900 C24's, and has only good things to say about them.

I recently looked at a used set of C35's, and a used set of Racing 1's (couldn't stretch to new for either). The C35's had a problem with one of the hubs, and the Racing 1's needed a new rim. The cost to repair either was prohibitive.

After a bit of research I ended up buying a set of Race 24's from Wheelsmith. They are a very nice set of wheels.
My only (very) slight issues with them are:
1) They look a bit plain compared to the Ksyrium Elites they replaced (I do like the look of Ksyriums with the machined surfaces and the nipples). Unfortunately the Ksyriums are now fitted to Mrs MdL's bike, so I see them most days.
2) They came in slightly over weight (1460g instead of the advertised 1430g, although the 1460g weight included the rim tape), but well within the tolerance stated on the Wheelsmith web site and certainly closer to advertised weight than any of the Mavic wheels I have previously bought.
Last week I took almost a minute off my previous best on the Col des Ares hillclimb course, and several minutes off my previous best on the Col du Tourmalet. Of course my recent interval training and the 300 miles charity ride I did the previous week may have helped improve my times, but the wheels were faultless. I am using 23mm Conti GP4000s, and on the 24mm rims they look huge . . . hopefully improving comfort and reducing rolling resistance.
Thoroughly recommended wheels.

I'd go with similar thinking to step-hent. Get some nice handbuilt wheels - my LBS does a velocity A23 laced to a hub that I forget right now, which come in around the same weight as the c24s but only cost around £450. Wheelsmith make something very similar too. Both should be more solid than the factory wheels, I think, and make you feel all smug and special when someone asks you about your exotic wheels.

I had C24s. I loved them to death - almost literally, as the rims have now almost worn out, one has developed a slight kink at the very top of the brake track (because the rim is almost worn through) and I've relegated them to rollers only. Booooo.

When they were still in service, they were light, zippy, smooth riding and all round brilliant wheels. The hubs are some of the nicest I've ridden - they were ridden through all sorts of crap weather and never serviced (accross two full season's use) and they never skipped a beat. They still roll ridiculously smoothly.

It's worth bearing in mind that one of the reasons they are light, though, is because there isn't much material in the rim. There's plenty of strength (because of the carbon reinforcement) but there's not much room for rim wear. Once the rims wear out, they can technically be replaced by Shimano, but its practically the same cost as buying the wheels again. It's the reason I now only ride handbuilts, and the only reason I wouldn't buy the C24's again - performance wise they were top notch.

If you'd consider handbuilts, the Dura Ace hubs would go very nicely with a Velocity A23 rim, and make a really nice zippy, lightweight, serviceable wheel. Probably on a par with, or cheaper than, the C24s too....

I like my Fulcrum Racing 1 2-way fit, and I run them tubeless. I've had absolutely no problems with them and they see to roll very well.
I've heard they are stiffer than the Shimano's, but can't compare as I've never had them.