Not to sound "jaded", but I boycott Supra and their boxes as much as I can get away with, as I find that they're a racket--just another way to sponge money out of Realtors' pockets.
Combination boxes are reliable, easy to use, inexpensive, and best--if a Realtor from another MLS wants to show your listing, you don't have to waste your time meeting them at the house to open the electronic lockbox. Combo boxes are also helpful if you have assistants or buyers' agents working for/with you, as you won't constantly be having to share your key, or paying for multiple keys.
Supra, their boxes, and their eKeys are expensive, and subject to malfunction. Some sellers will demand that you use them, however, so its a necessary evil.