Depends on what you are seeking. The GH has three features that are unmatched: It works by convection, it is extremely stealthy, and it heats up quickly. It's something you can easily carry in a pocket all day and use very discreetly when out of the home.
For those three qualities in one package, it pretty much stands alone. There are two other devices I'm aware of that feature true convection and ultra-fast heat-up — the Tinymight and the Airvape Legacy (currently my favorite). These devices are portable, but not stealthy and easy to use on the run in the way the IO is.
For stealth, there is the Da Vinci Miqro. It's a superbly made device that heats relatively quickly (35 seconds), and actually surpasses the IO in terms of stealthiness (you can literally hide it in the palm of your hand). Also, like the other devices, it doesn't smell much and the vapor is pure and largely dissipates in the air. So on the stealth and ease of use scale it's right up there. But it is a purely conduction device, and the chamber must be fully packed for it to work to its potential.
For equal performance alone, if portability outside the home isn't as important to you, the two convection devices mentioned are really excellent. Their build quality and customer service are at the opposite end of the spectrum from the Grasshopper.
Good luck!