Most of the dozens upon dozens of smartphone mounts that I see each year hold your phone in place with adjustable arms. However, Bracketron's latest Earth Elements (E2) series of smartphone mounts replace physical grip with the magic of magnetism. Mounting and removing a phone without fiddling with ratcheting arms is extremely convenient, but not without a few hidden trade-offs.
E2 series magnetic head
At the core of the E2 series is the magnetic head. This little hunk of plastic features a face covered with rubber tread to both protect the phone being held while adding a bit of friction to the mounting surface. Behind the rubber is a strong magnet that serves to actually hold the phone in place. At the bottom of the head, a pair of plastic tabs flip down and extend to form a base and add stability.
Each E2 series mounting kit also includes a trio of "Fe" (taken from the periodic symbol for iron) metal plates in two sizes: large and small. One of the large and the small plates feature 3M adhesive pads used to affix the plate to the phone or device. One of the large plates is included without any adhesive.
Users are instructed to adhere one of the Fe plates to the back of their device. If you're one of the vast majority of users who carries their phone in a case, one of the plates can be sandwiched between the phone and the case. Then just place the plated phone against the E2 head, which will hold it in place by magnetism.
Most phones already have metal internals, and the magnet is strong enough that it was able to hold my test phone, an LG Nexus 5, in place without the aid of the plate. However, the addition of the Fe plate makes the mount extremely secure. Holding the cradle by its base and shaking gingerly, I found it difficult to shake the phone free from the combination of magnetic and rubber grip. I think it'd take a severe jolt to knock the phone free once mounted in a vehicle -- at which point a loose phone is probably the least of your worries.
I can't think of a single person who would want to glue a metal plate to their precious handset, so I'm only really considering the E2 mount to be ideal for users with cases. I found that the Fe plates seem to work and fit better with thin, flexible cases and conform better to phones with mostly flat back panels. My Nexus 5's slightly convex back and extremely close-fitting hard plastic case required a bit of pressing to get everything locked in.
Phones like the LG G Flex or Samsung Round are probably out of the question without the user prebending the thin metal plate.