We reviewd the MLX against the 7600 and M320 many years ago. These days it would be the MLX against the ASR9000 and MX240/480/960. It didn't have the feature set we needed, but that was a while back.
Our national exchange point have been happy with them, using VPLS to run the fabric (I think AMS-IX do the same, too). But that's a relatively simple deployment.
I know some large carriers using them extensively, but not intimately enough to tell you whether they're really happy or not.
Mark.
You might get by these days at a peering point with something smaller if you are a smaller network and don't need a lot of 10G. Something like a Brocade CER-RT series. A 1U box with 136 Gbps of throughput that will handle 1.5 million v4 routes in FIB and 256k v6 routes. Sips power, doesn't take up a lot of space, has up to 48 GigE ports but only 2x10G. If they had a model with 6x10G, it would be a killer little box. It is basically a 1U MLX.