Alex,
Will any shortened antenna work as well as a 'full sized'
one? Not really, but it sort of depends on how much it's
'shortened' too.
Would this "shortened" doublet work well as a vertical
'dipole'? Probably as well as any 1/4 wave vertical with
the usual ground system (the 1/4 wave would be slightly
shorter, though).
Is this antenna worth building? I think any antenna is
worth building/trying if you like 'playing' with antennas.
Is it 'as good' as a 'real live' 40 meter dipole? Not really,
but then that depends on your circumstances. If you don't
have the room to put up a 'real live' 40 meter dipole, it'll
probably 'work' very well for you (anything's better than
nothing, right?)...
'Doc