Should you want to, getting into the One is relatively easy. The button high on the back of the case releases the fan and grille on the top. From there, you can remove a couple of screws and remove either side panel. That part is super easy. The problem for the inquisitive is that the radiators mounted to the side panels use extremely short hoses.
That's great for an extremely space-constrained system like this one, because it means you don't have a bunch of extra hoses blocking airflow. However, for taking the machine apart it becomes a problem. There's not even enough hose to fully pull the side panels away from the PC. I didn't have the One for very long, so I didn't want to risk breaking anything, and for that reason I didn't fully disassemble the thing.
Suffice to say that even though it uses many off-the-shelf components, actually adding or removing devices to the One is not a simple task. That disappointing unfriendliness to DIYers seems to contradict the fact that Corsair claims user-upgradeability was one of its primary design goals for this system. Even if we take the company at its word, there's not that much one could swap out inside the One without a lot of frustration.
The storage device is probably the most ready candidate for replacement or augmentation, in any case, so we explored that possibility. The One that Corsair sent me included a single storage device: a 960GB Corsair Force LE SATA SSD in one of its 2.5" bays. The machine has another 2.5" bay, as well as an M.2 socket hooked up to 4 lanes of PCIe 3.0. We appreciate the terabyte of solid-state goodness, but we also have to wonder why Corsair couldn't have tapped one of its high-end Neutron XT SSDs instead. This is a pro machine by its own admission—we'd have expected top-shelf stuff all around. The Force LE series is Corsair's entry-level TLC drive, and while it probably won't make a difference to most users in their day-to-day, it's the principle of the thing.
In any case, making use of this room for expansion means awkwardly unmounting one or both of the closed-loop liquid coolers while carefully holding the radiator in place to prevent it from tugging on the hoses. Having done similar things while working on my own custom loop in the past, that's not a particularly appetizing prospect. In particular, using the M.2 socket means removing the entire graphics card, because the slot resides on the back of the motherboard.
In general, the One does use standard parts: an SFX power supply, a reference-PCB graphics card, and Mini-ITX motherboard. If a part did break past the end of the One's warranty, the enterprising PC builder could replace it with some care and patience. Despite the company's claims of DIY-friendliness, however, we wouldn't want to dig into this box unless it was absolutely necessary.