Graphic LCD

Other than the required voltage levels, what is the difference between serial and parallel graphic LCDs? In which situations/for what uses would one be more desirable than the other? If I had to choose one, which is suggested? Also, how are LCD backpacks used? Are they a necessity or a convenience?

what is the difference between serial and parallel graphic LCDs?

The number of wires needed to control them. Serial LCDs require one wire, besides power and ground. Parallel LCDs require between 14 and 200 it seems.

In which situations/for what uses would one be more desirable than the other?

If you are short of pins, or just don't like connecting all those lines, use a serial LCD. If you like rats nests and using lots of pins, parallel is for you.

On the other hand, parallel LCDs are cheaper.

If I had to choose one, which is suggested?

Depends on your needs and resources.

In my limited experience (which includes using one serial backpack) a backpack is really another processor that is connected to a parallel LCD. You communicate with the backpack over a serial line and then think of the LCD as serial.

If the LCD is outboard, away from the rest of the hardware, a serial LCD is convenient as you only have to run 3 or 4 wires. I have epoxy coated a SparkFun serial LCD and have it fully waterproof. It would be a lot harder to do with a parallel. However, I have pretty much accepted the version of code on it as permanent. They do release upgrades/fixes for backpacks.