He are you finding the lightweight clinchers?
Happy to give my thoughts. Compared to the Enve 3.4 wheels, the Meilensteins are noticeably stiffer. The Meilensteins are also noticeably narrower. Tires wrapped to the Meilenstein appear much thinner than when on the Enves. This all translates as you might expect.
A digression on tires: When I first rode the Lightweights I was running an Attack/Force combo (22/24). The front tire was so narrow and the wheel so stiff it was like riding on the blade of an ice skate. Super stiff, pretty twitchy, and not at all comfortable. I swapped to a Force/Force set up and was happier with the cushier ride, but just didn't like the feel of the tires. The contis just seemed more harsh and actually seemed to provide less traction than the ultremos. I switched back to Ultremos -- a 23 on front, 25 on back. The ride is now perfect in my opinion. The Meilensteins, as they are a narrower and stiffer wheel, just call for a tire with a more bulbous/thicker profile and more supple ride. The Ultremos are much better in both respects than the Contis. I can't recommend them more highly to pair with the Meilensteins. With the Ultremos I also feel more confidence in cornering and better traction as well. Just all around a great matchup.
Back to the wheels: The Meilensteins are lighter and stiffer than the Enves. They seem to accelerate a bit better as a result. I don't notice a major difference in crosswinds, although the Enves may be a bit more stable. Over long rides the Meilensteins are slightly less comfortable comparing apples to apples in terms of both a Ultremo 23mm tire. This is likely due to the increased stiffness and the slightly narrower profile.
The awesome thing is the weight loss. I started with a 15lb, 4oz bike. Then I traded my Campy ti compact crank for an SRM power meter and gained some weight, but swapped on the Meilensteins. The weight loss of the Meilensteins was so dramatic that the net effect of the two changes combined was to put the bike just under 15lbs. Just incredible.
So far I've taken the Meilensteins for about 8-10 rides varying between 25 and 65 miles. With the Ultremos they are great all-around wheels.
The only situation I might favor the Enves is a long flat 100mi+ ride where I know there is quite a bit of rough pavement. The replaceable spokes (durability concerns - getting stuck with a worthless rim), slightly more cozy ride, and the possible aero advantage in the wider profile rim (no clue how much the advantage is?) might be worth the weight gain and the loss in stiffness. For anything less than 100 miles, any group ride, anything with a lot of climbs, any technical rides that require a lot of accelerations... basically most things, I would see advantages with the Meilensteins. Plus, I'll be honest, they look pretty sharp.