Keith McMillen Instruments QuNexus by undertone
I'd been looking for a professional standard portable keyboard that I could throw into a backpack or luggage, with a pro build meaning that its performance should be reliable and predictable. I've bought and tried cheaper USB keyboards, always to be disappointed especially regarding velocity performance.
Then the Keith McMillen QuNexus was brought to my attention, which has a number of very desirable features including: class compliance (ie iOS/OS X/Windows driverless functionality); velocity and Poly AT;no real moving parts that can break; a MIDI channel rotation mode allowing for every played note and its data to be output on a separate MIDI channel; CV interfacing (for Modular or older analog synths); a useful (and pretty cool) LED distribution; Mac/PC software for extreme customization of the keyboard's behavior... you can read all about this and more on the author's pages and download the user's manual for details. It's a real powerhouse of useful features, and in principle these are worth the extra money you pay for it.
So I went and bought one from a local retailer because I didn't really feel like waiting for one to be shipped, even if it would have saved me a few dollars. I plugged it to my iPad (via the Apple Lightning to USB adapter) on which I have quite a few softsynths and it was instantly recognized. Very nice... except... I kept running into 2 notes that didn't seem to respond to velocity like the others. "No problem!", I thought and downloaded and installed the editor which gives you access to customizing the velocity on each key. But even if I boosted the velocity sensitivity of these 2 keys, I simply couldn't get them to match the other keys.
So I brought it back to the store along with my iPad and Lightning to USB adapter, discussed the problem, and then proceeded to try out each of the other 3 units they had in stock: the first 2 had similar problems on other keys (one had more than 2 keys with the same issue) but on the 3rd and last one all the keys were pretty much the same. Phew! I actually wanted the QuNexus so it was a relief to find a decent one. It wasn't clear whether the store was going to return the units or not. I hope they do...
1:4 is a pretty bad ratio and points to unpredictability in the manufacturing and/or poor Quality Control. Either way, I don't recommend you buy the QuNexus online: make sure you can try the unit you're going to buy or have a direct and quick line to exchanges or refunds. It's rather sad IMO that in this day and age that you can't trust the quality of a product to this degree. I would stay away from eBay as anyone with this problem might not mention it in their copy and retailers won't typically test the units they're selling. All this is the reason I put "Bang for buck" at 3 stars.
Other than this issue, the only other issue for me is the pitch bend control, which initially feels like it's really uncontrollable. But with some familiarity it feels like it could be controlled reliably, although I'm not there yet; it shouldn't be so hard to control, but you have to consider that this keyboard is fundamentally different and requires adapting to new playing techniques. And like any keyboard without some kind of display for parameter changes, some things may be cryptic unless you're using the setup software, which is why I knocked one star off of "Ease of use". BTW, the QuNexus doesn't handle direct MIDI 5-pin DIN connections; if you want to do that you'll need a separate $50 USB-to-MIDI conversion box.
If you do your homework and test the unit before you buy, I think this is really an excellent little tool. No one can complain of its portability. I had found a forum discussion (which sadly seems to have disappeared) about the QuNexus' uneven keys where someone did exhaustive tests on different units and found quite a variation of behaviors between keys. I don't know if my simple playing test also would weed out these reported AT problems. And there's also the possibility that over time or hard playing, some keys might start responding less... that's pretty much a "to be continued..."
So caveat emptor: it's sad that in 2015 we still can't get someone to build a small, performing, and ultra reliable portable keyboard at a reasonable even if premium price. But considering its long list of features, once you've found the pick of the litter, it's a pretty sweet little controller.