The nanoPad, which I reviewed several months ago, had 12 pads which was more than enough to cover Redrum's 10 channels. Given that the nanoKey is just a portable two octave keyboard, there are no Reason-specific advantages or limitations to using the nanoKey. It "fits" all of Reason's instruments just fine, so its effectiveness really comes down to how it works as a keyboard, so this review will concern itself primarily with that, rather than any specific issues with regards to using it with Reason.
While the nanoSeries saw a lot of positive buzz when it was first announced, the nanoKey was largely seen as a disappointment when it was released. The keys are responsive and pressure-sensitive, but as you probably already know, have more in common with laptop keys than any musical keyboard, making them a little awkward. If you're used to hammering out a few notes using a "virtual keyboard" like the one in Logic (and now Record, Propellerhead Software's newest app), the nanoKey will probably feel comfortable, even spacious, but if you're more used to a traditional keyboard, or possibly even a mini-keyboard like an Oxygen 8, you'll find the nanoKey a bit lacking.
The the left of the keyboard are six buttons which control Pitch Bend (up and down), Mod Wheel, CC Mode, and two buttons to toggle up and down in Octave. These keys feel like the keyboard keys, so again you have that laptop keyboard feel, but don't appear to be pressure or velocity sensitive. The Mod Wheel button, for example, is either on or off, with no variations between, making its use rather limited. The same thing is true of Pitch Up and Pitch Down, so while its nice to have those options, they're not very practical in "real world" use. Using Korg's Kontrol Editor you could certainly shorten the range of these keys, so that your Mod Wheel goes from 0% to only 40%, instead of 0% to 100%, for example, but because you seem to be limited to either on or off, your "performance range" will be limited to whatever presets you design for your gear.
Of course, having said all of that, even with access to a few other keyboards at home, I tend to use the nanoKey quite a bit, as its simply more convenient. I think I've mentioned it before, but my "studio" is also my home office, so there's usually a bit of setup time involved with getting out gear for music making, especially if I've moved away from the office to the kitchen table or patio. The nanoKey effectively removes that barrier for me, as its small enough to just pull out and use, without hunting for the wall warts and USB cables needed to get my Oxygen 8 or Yamaha keyboard setup and ready to go. No, it's not as nice as even my first generation Oxygen 8, especially if I'm hoping to add a little more human performance to my playing via the mod or pitch bend wheels, but its immediate, which tends to suit my needs when getting down an idea quickly.
One point of interest is that the nanoKey is by far the thinnest piece in the nano series of controllers, about half of the thickness of the nanoKontrol, and slightly thinner than the nanoPad. The differences don't seem like much when you look at them, but even as slight as they are, the nanoKey feels like the most portable of the three units. I packed my nanoKey in my bag today, along with my laptop and my usual work stuff, while out running errands and it took up very little room, so little, in fact, that it might find a permanent place in my bag during my long commutes.
So, do I recommend the nanoKey? I do, but with some caveats. If you want something portable and fast to set up, the nanoKey can't be beat. Korg is marketing the entire nanoSeries for use on the train or bus, in-between classes, or wherever else their cartoon spokesman needs to make music, and it suits that grab-and-go mentality very well. Its the very definition of "quick and dirty." What you get in exchange for that portability and size is unnatural key action and almost useless pitch and mod capabilities. If that's a deal-breaker for you, and I would guess that for a great number of you it is, you should look elsewhere.
Note: The nanoKey uses the same USB driver as the nanoPad and nanoKontrol, so if you already own one of the nanoSeries devices, your drivers will already be installed and you should only be required to add it as a new device in Reason.