We rent several different MacBooks and have not had any major problems with any of the back systems. Recommendations would be use AC power when available, change batteries when you get down to about 1/3, have additional batteries or external battery available, use 15' or shorter FW cable, max out RAM and upgrade Video Card where you would have 512MB. We will often take out MacBooks on initial camera back demonstrations so we don't have to worry about the clients system or installing software and leave files with the clients, have not had any problems.
Best,
Rich Andres
Foto Care
41 West 22nd Street
NY, NY 10010
212-741-2990