I am interested in upgrading my wheels to a good mid-level but would like input. My current bike came with Mavic Aksiums and I would like to upgrade to something lighter and/or more aero. The region I ride in is mostly flat. Typically I ride 3-4 group rides a week of 30-50 miles @ 18+ mph. I have been entertaining the thought of either upgrading to Krysium SL's, or on the other end, something like the Sram S40 or S60. I have been following the bike build threads and these wheels have all been suggested. It would be helpful to know what people think of either of these choices or can refer any good reviews for wheels.
Are you set on pre-built wheels? If not, why not get some wheels built.
I'm just building up a set of wheels for my cross bike. I'm using Campag Daytona hubs (brand new - but a discontinued model - same as Record just with a heavier freewheel body), Open Pro rims and a pretty heavy gauge Wheelsmith DB14 spoke (since they're going to get a hammering). Total cost is about C$450 - and total weight is 1620g.
If however I was just building for road, I would use DT Revolution or similar, which would drop the weight down to about 1530g - at the same C$450 price.
Compare this to a set of Ksyrium SL's - 1485g and about C$1006 if I buy on line or C$1300 at my LBS ... That's a **** load of extra money to save stuff all weight ... Jeez, for that much price difference, you could get yourself a set of DT Swiss 240s hubs (and still have change) - I have the dirt version on my mountain bike and they are just gorgeous! Add to the cost difference the fact that hand built wheels are generally stronger, more durable and easier to maintain than factory built wheels and you understand some of why I like the hand built way ...
Regardless of which way you go, I'd suggest that your main point of focus should be getting something with good/fast rolling hubs rather than lightweight as your top priority. You say that you ride mainly on flat lands, so you'll notice the former far more than the latter ...