I have had 2 sets of Ones and now I am on Hopes. Both have their good and bad points. But either way you go, you are getting the best brakes made IMO. I have a set of M4s and a set of V2s.
The ones are more versatile, meaning they fit a larger range of bikes from aggressive XC to DH. They modulate superbly and have lots of power. When I owned them, pads were difficult to find and were pricey, this no longer seems to be the issue. My biggest issue with them was the lever action was very inconsistent, especially on longer, steeper hills. The lever would pull normally on one stroke, and the very next would come to the bar. This occured even after Chris at Formula replaced the seals and bled the brakes for me. The inconsistency drove me nuts. It is my understanding that it was a lever issue, and one of the reasons the new lever was developed, so this may now be a non-issue.
The Hopes on the other hand are heavier, and more purpose built. The M4s are the classic AM brake with lots of modulation, and power that is similar to the ones, just later in the lever stroke. They are a work of art. built like tanks, and based on my reading, they will last for many years. No special kits are required for bleeding, and bleeding is flawless. I find the floating rotors to be more durable, easier to true, and make a cool as hell ping when they are hot (so you know they are doing their job). The lever action is ALWAYS consistent, hot or not. The lever action is just fantastic and the BPC really works with a great range.
The V2s are in another league alltogether. They blow the ones away in power, have beautiful modulation with sintered pads, and are not phased by anything. They are heavy though, but in their intended application who cares?
My only real issue right now is with my V2s, they are noisy regardless of the pads I run. The Formulas are dead silent. I will work with Hope to try to isolate the issue now that I have done the basic troubleshooting.
I have also had a chance to play with the Elixirs and they are solid. My only real issue is the CPS mounting system. It is very inconsistent, causes strange issues, and the pieces actually wear causing other issues. The lever action is very solid and the power is crazy.