what are they? do they replace an amp head, can you use a pedal with them to swith between fx, can you make and save a certon sound on them,are they like a minisound lab?
It depends on the rack unit. Some are preamps, some are effects processors, some are power amps, some are nothing more than drawers to store stuff in.
ESP LTD EC-256 and a Fender Deluxe VM
yeah it really just depends. i have a rack tuner which makes it way easier to tune between songs because i dont have to switch and pedals on and cut my signal. just pluck the string and tune away. then my power conditioner just gets rid of all AC noise that would interfere, and my sonic maximizer just beefs up my sound.

its all generally ran through your effects loop and doesnt get messed with during the show.
Schecter Omen 6
Agile LP 2500
Kramer RR V
Peavey 6505
4x12 with Eminence Screamin' Eagles
Mogami Cables
Korg DTR-1000
Furman M-8 Power Conditioner
MXR Smartgate
BBE Sonic Maximizer
My Band