The instrument sounds great in his hands. But the way he plays it--held chords in the left hand, fills, endings, etc., is EXACTLY like an arranger. The videos prove the Montage CAN be used as an arranger. It also begs the question, if you are going to play it this way, why would you choose it over a Genos?
Every chord he triggered by pressing at least three keys, and in most cases he moved around to reach a specific inversion. Personally I would like to know whether the arps in the Montage can follow simplified partial chords (such as just the root and seventh below), but that's not demonstrated here. The style patterns are all held, so it allows him to "stab & move" just like most arranger players do. With all of the movement and button-pushing in these demos, I wouldn't call this an "easy play" instrument. Seniors trading up from their Lowrey should look elsewhere.
The sound samples are great, but it's hard to say whether they're really better than what's in the Genos. Lead sounds are very important. Style sounds not as much because they tend to get lost in the mix. As far as the "arps" he's using: they don't sound like arps I've heard before, they sound like style tracks! I would say the main reason to buy a synth is because you WANT to create your own waveforms, and generate arps to create electronic or dance music. If your goal is a decent cover of Moon River, you might eventually get there but this is doing it the hard way. And if you sometimes want to play arps, I think the Genos has a few (admittedly it's a token functionality compared to what the Montage and MODX can do.)
It's always been possible to "assemble" your own custom style on Yamaha arrangers by copying tracks from various factory styles. In some cases you might need 3rd party utilities on your PC, or it might be easier to do it that way. No matter what it will take some time. Because just like on the Montage you would have to audition the parts individually, and then see how they harmonize with the other tracks you selected for your user style. Like anything else in life, you'll get out of it what you put into it.
Which brings us to the question of content. Everything he played in these videos had a "modern" feel to it. But what if you downloaded the styles from the MM6 (which was really an arranger) and installed them in your PSR? Or "patterns" from the DJX-II? Check out this page:
http://sandsoftwaresound.net/mox-perf-to-psr-style-part-1/The author Mr. Drongowski also posts on PSRTutorial. He figured out how to take the arps from the MOX (the MODX's grandfather) and convert them to arranger styles. If you don't have time to do it yourself, you can just download the zip file at the end of the article.
The Montage, or even the Motif wasn't Yamaha's first music production synth to have arranger-like functionality. The QY-700 allowed you to assemble a "pattern" (style!) from a bank of stock phrases. Sound familiar? It also had real time chord recognition, and this was back in 1996! But how many just went out and bought a PSR-730 instead? Bottom line: What songs do you want to play? What's the end product you're trying to achieve? Once you know that, there's undoubtedly more than one way to reach that goal. Choosing the right tool (and populating it with appropriate content) will help you get there faster. More joy and less frustration. My $.02.