Although you may not be ready to admit it, the iPhone is a professional video camera for many shooters. We use ours with a handheld gimbal at almost every shoot to get super steady moving shots. Feiyu Tech recently sent us their SPG Plus two-handed gimbal, and I tested it out.
The build quality of the SPG Plus is superb. The entire thing is made from aluminum, and the hand grips are coated in some sort of ultra soft rubber. Using the gimbal is straight-forward: simply slide your phone in and balance it out using the toolless knobs, hold the power/mode button for three seconds, and you're up and running.
The gimbal comes with an app that allows you to customize the gimbal and record video using the physical record button on the handle. I personally didn't find the video app robust enough to actually use, and therefore my record button will probably be forgotten. The thumb dial feels great and is extremely easy to use, and the mode button can be pressed to switch modes easily.
Some videographers believe that two-handed gimbals are more stable than single-handed versions, and that may be true for larger, heavier, cameras. But I personally think that using two hands with an iPhone is overkill. What you're really paying for with this system is the ability to attach accessories such as lights or microphones to the top of the gimbal. If you need these mounting points or if you really prefer using two hands, the SPG Plus is a fantastic product.
The SPG Plus costs $300, and that's a very reasonable price when you consider the build quality; but there are some other options on the market that are significantly cheaper. I personally don't need the mounting brackets and I prefer the single-handle design. Because of this, my favorite iPhone gimbal is still the Smooth C. It's much smaller and it's almost half the price of the SPG plus.