The units will have different movement speeds though... Both inherently and due to terrain (rivers, roads, hills and forests in various combinations). I don't see a way to avoid this problem... Unless all units will move at the speed of the slowest one, so typically 1 tile at a time, while using the formation command. Even then, mountains will totally disrupt the command because some units will be forced to stop or 'run into' other units as they try to circle the mountain range...
Fwiw, the formation mechanic was only mentioned once in one of the earliest previews, and from the wording it was unclear if the previewer had it mixed up with the way that armies and support units work now. It's more likely that it does exist (imo), but only slightly (I'd say the odds are something like 60-40). Here's hoping that they somehow sort it out...
Even if it can't work on land though, I can see the formation movement working on water tiles, because there are no mountains and the terrain has no movement modifiers (presumably). So the only changing factor will be the movement rate of the units themselves; if they'll all be moving at the rate of the slowest unit, then this could work for fleet movement (and embarked units).