We are running Marathon EverRun FT for our Exchange server.
We have 2 identical servers whcih replicate each other, they both host a virtual server (it is the same instance of the server so when you move the mouse on one it moves on both etc.) which has Exchange installed, fault tolerance wise it is superb, can handle anything for a NIC, CPU, Disk to entire server failure without users even knowing. Before I go off on pros/cons, which version are you looking at as we are running EverRun FT 7.1.1 which is completely different to the VM version which is Citrix XenServer based I believe.