Ian Cutress, Anandtech's Senior Editor - Motherboards, recently wrote an excellent in-depth article on the ROG brand, so I would encourage you to read that for a detailed analysis. JJ at Asus also gives a detailed overview of each ROG board on Asus' YouTube channel, so you should look those up, too, for more information.
Here is my perspective. All three of the Maximus V boards (Socket 1155 / Sandy & Ivy Bridge) have similar features and components, but they are targeted towards slightly different market segments. The Gene is an entry level board for the ROG series (<$200), with a MicroATX form factor. It is more focused on gamers and people who want a small form factor for a small case, but like all ROG boards, it is also highly overclockable. The Extreme is about 1 inch wider than ATX, so it wont easily fit in a lot of mid tower cases. It is packed with extreme overclocking features, but is also an excellent board for gaming. These two boards are at the opposite ends of the ROG spectrum. Gene has onboard audio (gamer focused), and Extreme has sub-zero overclocking features (overclocker focused). The Formula sits right in the middle of the ROG product line, incorporating some features of both (e.g., onboard audio and extreme OC). Formula is only 1/2 inch wider than ATX, so it can fit in many mid tower cases that won't fit an Extreme, and it has additional PCIe slots that the Gene's smaller form factor can't support. So it is sort of a compromise board, targeted at both gamers and overclocking enthusiasts.
Personally, I like the Formula best for my needs, since I am mostly interested in gaming, I like the onboard audio, and I want the additional PCIe slots in a form factor that will still fit in a mid tower case.