I got some "star" key rings for free from the dealer when we bought the SL. We got enough of them to put each spare key on one of those fobs.
The others "star" key rings I purchased myself prior to that.
I don't like to keep a bunch of keys in my pocket, so just a car key and a house key on the MB key fob for all the cars.
The wife doesn't like to sort through the assortment of keys, so she keeps our house key, a neighbor's key, and all of the car keys on one (Dooney & Bourke) key fob.

Since two of the cars have those bulky "switchblade"-type units, it's a busy looking set!
I want to get those die-cast miniature key fobs of the E, SL and ML, and keep one primary key on each one. Then it would be easy to identify which set I am looking for...