Well I popped off my spacebar to get a feel for this type of layout.  I like some aspects but not enough to outweigh all the cons, most notably is that this is a hardware mod and would prevent me from using it on friends computers.  Just imagine, "hey dude.  Can I use your computer?"  "Sure. ... Hey! Why the hell are you prying off my spacebar?!"   LOL.

I did find your choice of home positions to be wanting.  I would move the left to QWFP and the right to UY;[   That lets my thumbs rest on/near V and M.  Your way produces a wider layout but makes it harder for my thumbs to reach more than 1 key without my wrists bending.  My way, I can easily reach CV and M<  There is still a stretch to BK but its no longer too bad.  I also find it impossible to hit ZX and >? with my thumbs no matter where my home row is.  I assume these keys are hit with fingers instead?  If so, thats quite a stretch and would make those keys next to useless for me.

The second biggest con I see with this is the key stagger.  From home-row to top-row is only a 1/4 key shift but from top-row to number-row is a 1/2 key shift.  Home-row to bottom-row is also a 1/2 key shift but the bottom-row is used less frequently so I find it easier to deal with if I use the normal home-row as opposed to moving everything up 1.

Another thing I noticed is that even though I'm using a wrist rest, I would often accidentally hit the Control keys with my palms.  Maybe I could train myself not to do that over time, but its something I'd rather not deal with.

I can see why someone would choose to use this.  It would be awesome to have backspace, shift, space, and enter under the thumbs.

I would assume that if you didnt take the spacebar off, that your thumbs would fatigue because each key-press would hit your target key and the spacebar.  Yes, the thumbs are strong, but they are now doing much more than just pressing space.  Its also harder to tell if you are going to smack an alt key by accident.

Maybe this is slightly old, but I'll give a few comments anyway!

1. This is a keyboard mod, thus not very suitable for laptops, sharing computers, etc.
2. I did have a go removing the spacebar and shifting keys up a row once. Cons:
    key alignment on a regular keyboard: top row is way off the usual
    one space bar key puts a lot of work on a thumb!

About wide boards in general:
3. Like ghen says, moving / is a real pain to get used to
4. Difficult to change between modded wide-board and usual board (I found worse than changing between colemak/dvorak and qwerty)

Sorry if that's very negative! However, for extra modifier keys: after getting used to it, I can comfortably use the alt keys from the standard hand-position, and on thinkpad keyboards (slightly shorter space bar) the next key to the right of alt is also usable.

@screaminglemon
Actually, with the spacebar removed your 'qwer iop[' (qwerty marking) home position is much more suitable for the thumb keys on the bottom row, and more comfortable overall. The problem lies in where you put the number keys. The numberblock was the best I could come up with.

@DreymaR
Leaving the Spacebar on the keyboard, setting it for a null value and then typing through it is indeed quite doable, but somewhat confusing to the fingers hitting two keys at once. I misunderstood you at first to mean typing OVER it, which would have been simply awful for keys as frequent as Space. screaminglemon's comment helped to clear that up for me. Even if I misunderstood that comment at first as well.

@Cyborg16
Odd. I never had an issue with switching from wide to normal boards as long as the key stagger was the same. Also, I never use my left thumb for space, so that is not only a non-issue for me, it is also my primary motivation for trying to develop this layout.

@ ... um, everybody

After actually trying to use these layouts, which is significantly different from just testing it, I made some changes. Here they are.

THE SPACE BAR STAYS!
That was one of the biggest gripes about the first version, and losing a key was pretty irritating. Turns out that previous recommened space location was awful, and the shift wasn't much better. Long story short; the new Space position is on R.Alt, and the Spacebar now functions as Shift. To make the new Space position comfortable, the R.Home position has been pushed up against the edge of the board. If you are a left or ambidextious spacer, this probably isn't for you. L.Alt is recommended for another high frequency key, as the thumb now has to hit the bottom row AROUND the Spacebar. Not too much of a hardship, but I wouldn't want to put E there.

The Staggerfix Variant
As has also been noted, adjusting to the upper row stagger is pretty daunting. Having tested it though, I have found the stagger on Left to be not so bad. It pulls the zxcvb into alignment and the upper row is fairly easy to adapt to, as it is usually counter aligned anyway. The upper row stagger on the Right though is very hard on the wrist. Since the upper right is usually in fairly good alignment, the hand tries to position itself in line with the keys on upper and home rows. With the dramatic stagger increase... yeah. So, this version perserves the right hand stagger, by only shifting the left hand up to use thumbkeys. The right hand remains on the home row and the right thumb is more or less dedicated to Space and the occasional Shift. (Though, I suppose you could switch this to fix the Left hand stagger instead.)

Note: Undecided keys left blank, but all key locations are negotiable. The Windows and Menu button have not been edited, as not all keyboards have them, or have them in the same place. These represent layout independant keyremappings, and as such I am not messing around with layers. Thumbs seem to rest around the qwerty C and Comma.

Colemak
___________________________________________________________
| ` | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | - | = | BSp |
| Tab | q | w | f | p | g | j | l | u | y | ; | [ | ] | \ |
| BkSp | A | R | S | T | d | h | N | E | I | O | ' | Entr |
| Shift  | z | x | c | v | b | k | m | , | . | / | Shift  |
| Ctr | Wn | Alt | Space              | Alt | W | M | Ctl |
-----------------------------------------------------------


Colemak using MK-Type V2.Staggerfix (Left thumb uses bottom row)
___________________________________________________________
| q | w | f | p | g | 7 | 8 | 9 | ` | [ | ] | \ | / | BSp |
|   A | R | S | T | d | 4 | 5 | 6 | j | l | u | y | ; | ' |
|    z | x | c | v | b | 1 | 2 | 3 | h | N | E | I | O    |
|        |   |   |   |   | 0 | - | = | k | m | , | .      |
| Ctr | Wn |     | Shift              | Spa | W | M | Ctl |
-----------------------------------------------------------

Colemak using MK-Type V2.Classic (both thumbs use bottom row)
___________________________________________________________
| q | w | f | p | g | 7 | 8 | 9 |   | j | l | u | y | ;   |
|   A | R | S | T | d | 4 | 5 | 6 |   | h | N | E | I | O |
|    z | x | c | v | b | 1 | 2 | 3 |   | k | m | , | .    |
|        |   |   |   |   | 0 | - | = |   |   |   |        |
| Ctr | Wn |     | Shift              | Spa | W | M | Ctl |
-----------------------------------------------------------

If you want Staggerfix for both hands... well I just posted ThumbsPlus over on the Wide-Mod thread. It gives slightly better thumb usage, but it is hardly ideal.