I just started rocking a mohawk last week as a way to not have regret once I go bald in my 30s. Anyway, 3 or 4 months ago I heard about the headblade on the The Big Idea with Donny Deutsch, and when I decided to go with a mohawk it was definitely a product of interest. However, I would have to give the product two thumbs down if you have a mohawk.
I bought the headblade at Meijer (a big box chain in the midwest), along with some headslick to help prevent ingrown hair and bumps. After removing the headblade from its blister pack and slipping it on my middle finger, as the directions instructed, I noticed the ring was far to large for the diameter to be placed between the recommended first and second knuckle. No problem, it fit like a charm between the second and third knuckle. I set the headblade aside and lathered up my head. The instructional video on their website stated for new comers it is best to shave with the grain. I slipped the headblade back on my middle finger and started shaving the top of my head. Not a particularly close shave, but I had let my scalp grow out for a week so I'll just go over it again. But first, let me move to the back, and this is where the headblade became difficult to use. The headblade rests along your fingers and your fingers cover the entire assembly, and this coverage makes it extremely difficult to see where the blade is in relation to the mohawk line. I decided to flip the razor to shave against the grain. Again, the shave just wasn't as close as I would have liked, and it made shaving the back of my head (the specific problem the headblade was intended to address) even more difficult. In a parnoid state I ended up not even shaving close to my mohawk line and had to go over it with my trusty mach3. For that reason I think the headblade will be useful when I start shaving my head completely, but as for shaving with a mohawk I will have to pass and got back to my standard handled razor.
If you're still interested in the headblade you can check it out at headblade.com