*Edit* So I have ended up getting the rear wheel rebuilt with new spokes and nipples because it kept slowly losing spoke tension over time... My local shop and Mercury both suggested that the spokes were likely not prepped correctly when first built. To fix the problem Mercury suggested putting blue loctite on the spokes, but my local shop said that would likely permanently freeze the spokes in the aluminum nipples and cause truing problems down the line. One rebuild and $150 later with slightly different spoke specs I hope the problem is solved. I still think these are good, stiff wheels for the price that likely add an aero benefit, but I am dismayed at the quality control lapse.
The brake pads also continue to squeal and wear down quickly; I plan to switch to SwissStop black prince pads. The stock pads have decent stopping power and work okay in the wet, but with each stop and squeal a small part of my soul dies. I have tried toeing the pads in with no improvement. Unfortunately, using different brake pads voids the warranty. To save my soul I plan to void the warranty and switch to different pads. Will update with results. FYI: Buying the pads through Mercury's site is a rip off, just search for equinox or gigantex brake pads online to find way better deals.
If your budget is tight I'd recommend these, but if you can manage to invest a little more I'd recommend something higher quality with better hubs like the November Rail.
This is my first set of full carbon clinchers, so keep that in mind when reading this review. I got my set of Mercury M5s off this site on sale for $1000. After 1800 miles and a successful Spring racing season, I definitely feel like I have gotten my money's worth. These wheels are noticeably stiffer/less flexy than my 1550g aluminum clinchers (both my Rol Race SLs and my American Classic Victory 30 tubeless). This is very important to me as a 170+ lbs sprinter, because I am able to flex a lot of wheels and I find it annoying. I also feel like these wheel hold speed better than my 30mm aluminum clinchers, although I don't have a wind tunnel to verify it. As far as cross wind susceptibility is concerned these do get pushed lightly in the wind, but the feel is similar to my dad's HED Jet 6 FR wheelset (which is supposed to be pretty good in that regard). Many of the smaller riders I know prefer 35-48mm wheels because it is harder for them to control deep wheels, while I see a greater percentage of larger riders on deeper wheels.
I have had some issues with the wheels, and if I did not get them for so cheap (relative to other carbon clinchers) I may have given then a lower rating. First of all, I had to get the wheelset re-tensioned after delivery because it kept coming out of true. It's annoying that wasn't caught in the factory. Second, the stock brake pads squeal and tend to wear down quickly. All in all I am satisfied with the wheelset and performance, especially considering the price.