I have used OceanReef Space Extender, Neptune III, and Kirby morgan M48. Here are my thoughts:
Oceanreef space extender and Neptune III: They fit the best for me ( I have no facial hair ). However, Neptune III breathe is probably one of the best reg i have ever used, Now I dive with a A700 SP, and the neptune III almost breathe as good as my A700. But Neptune III front case was not secure when I received the product to the point where it jiggles, probably QC wasn't good. So I deceided to sell the mask and got a oceanreef space extender.
Space extender on the other hand does not breathe very good IMO. And also, I had a "close call" on one of the boat dives -- I enter the water first and waiting for everyone else to join, but the CO2 build up was so bad on the surface with a surface breathing valve open, I had to take off the mask to be able to get a fresh breathe of air. Also, my air usage went up a lot. The only benefit that I can see from a FFM is field of view, be able to add on communication and use your nose to breathe.
I also have experience on the kirby morgan M48, and this is probably by far the best FFM on the market IMO. This mask is modular, meaning you can take the bottom half of the mask off while you are floating on the surface. This gives you talking ability and no CO2 build up on the surface. M48 also breathe very very good with the reg. The only problem I had with the mask is the equalization was kinda strange, so I had to get used to that.
When dive a FFM, here are couple things might help you
1. Try to enter the water last in a group dive, because it is very uncomfortable to breathe through a ABV the whole time on the surface ,( both fogging and CO2 build up issues)
2. Must carry a spear mask with you, because i almost had a situation where I had to bailout from FFM. Also, practice being neutrally buoyant while switching from a FFM to a regular mask.
3. Because of the large volume inside of the FFM, you might need to adjust the weight to compensate that.
4. It is difficult to talk to your buddy with a FFM on the surface if there is a problem with your gear and your buddies gear, or just general discussion of the dive. So try your best to resolve all the issue before you put your mask on. (M48 is not a problem regarding this issue)
That's just some of my thoughts and experience with FFM, hope it helps you to decide FFM purchase.