It's difficult to get really excited by the Nikon D610, but that doesn't
mean its an unexciting camera. The 610 might be 99% of the older D600, with
the same sensor, but it is a better camera than its nominal predecessor, and
a tougher rival for its main competitor - Canon's EOS 6D. A speed increase
of 5.5fps to 6fps isn't much to get excited about, but this slight boost,
plus the new quiet continuous shutter mode (which to be honest is just '
quieter' - not silent) bolster the D610's enthusiast credentials and as such
make it a more serious option as a second body alongside a D800 or D4 to
professionals (especially event and music photographers). Both of these
improvements are made possible by the creation of a new shutter mechanism
which – although Nikon is tight-lipped on the matter – we hope (and
strongly suspect) will eliminate the notorious sensor oil issues experienced
by many D600 owners.
In summary, we don't expect that D600 owners will start selling their
cameras in droves and buying up D610s. Unless you really need the quiet
shutter options (or if you're still having persistent issues with muck on
the sensor) there's little reason to upgrade. But for APS-C photographers
who've been hanging on before moving to full-frame, or for professionals
wanting a second full-frame body, the 610 is a stronger product than its
predecessor, and one that we suspect from our experience with the D600, will
perform very well.
As always, our final judgment is reserved until we've been able to go and
and do some shooting, but we're certainly looking forward to getting a
production quality sample into our studio.