The Arc Touch derives its name from its two main features. It should be obvious that the 'Arc' part of the moniker comes from the shape of the mouse. Although that's not entirely true, as the Arc Touch's piece de resistance is that it collapses flat making it, says Microsoft, easier to transport than a less flexible mouse (flexible both metaphorically and literally). The 'Touch' aspect is the middle 'button' of the Art Touch, which isn't a physical button, but rather a touch-sensitive strip.
The Arc Touch's folding mechanism feels surprisingly solid; there's a tangible and audible click when opening up the mouse. Cleverly, locking the Arc Touch open also turns it on (and flattening it, of course, turns it off). Admittedly this does mean you can't leave the Arc Touch on a desk in its active configuration unattended as the battery will drain unnecessarily, but this is a minor concern.
A useful design consideration is the magnetic patch on the underside of the Arc Touch, to which the devices transceiver can be affixed when not in use. Even though these are generally intended to be left plugged in - hence the small size - it's good to know that it shouldn't get lost if you do chuck the mouse and transceiver in a bag.
Although it's fairly small, the peak of the Arc is high enough to rest your hand on, and although it's far from as comfortable as an ergonomic mouse, it's fine for occasional use. If you're looking for a mouse to whip out for the odd half hour or so on a train to and from work (and travel on a magical train that affords you the space to use a laptop and mouse) you shouldn't have any complaints, but use the Arc Touch for longer periods of time and you'll find yourself longing for a more comfortable pointing device.
Another potential issue arises if you put too much pressure on the top of the Arc, as it will collapse - the locking mechanism isn't strong enough to take much weight. Most in the TR office didn’t find this a problem, but it's something to remain conscious of. Furthermore, we remain unconvinced that the folding ability of the Arc Touch makes is that much more portable than other small mice. If using it in accompaniment to a netbook, you might just be able to squeeze the Arc Touch inside a neoprene sleeve, where a less compact mouse wouldn't fit, but otherwise its being able to flatten probably won't be a major advantage.