I have used both the lcd graphics OHP panlel (that sits atop the OHP
stage) and also an lcd MM projection unit that looks about the size of a
large attache case and contains its own projection equipment. From my
perspective, the MM projection unit is far superior in every way.
For the OHP unit you must have a very bright OHP system, and sometimes the
special (halogen?) ones require some time to warm up to working
temperature. Moreover, the panel itself doesn't always fit very well on
the stage (cuts off parts of the display), or if you are doing something
else on the OHP you are constantly re-focusing (unless there is a
supplementary lens that will swing out of the way - this isn't bad), and,
finally, I have seen one of these things inadvertently pulled off the OHP
stage by one of the various cables (power supply - line to the monitor,
line from the monito, etc.). After the crash there was much silence,
except for the quiet weeping of the presenter.
The MM projection units come nicely packaged and can be used independently
of any OHP, I find that there is just one main power line and the display
cables. The one that we have will also allow video signal input, so you
can play VCR outputs through it. It also has an amplifier for MM sound,
but I haven't used that (yet). The system closes up well to protect the
internals and is actually smaller in size than the OHP panel in its
carrying case. I have no idea how sensitive the optics are to jarring;
this may be a consideration. I still like it over the other OHP panel.