I have to slightly disagree with most of the reviews here, I believe. The piano sounds in the RD-170 to me sound a bit muffled (not just "darker") and lack some power in the higher registers. From the C' and up they're rather thin, though adding some string resonance helps a bit. The same goes for the Rhodes and other EP sounds, the Wurlitzer in particular is ridiculously thin. They also have a really oddly short sustain, even though there is very obvious and audible sample looping. To be fair, the RD300 and even the RD700 have the same short sustain, though the tone there remains thicker.
Now for some balancing the good and bad - none of these problems are very serious for live cover band use, which is what I almost solely use mine for. I would have liked for the piano to be a bit more dynamically responsive and to have more bite especially in the solo registers (exactly where it lacks some now), but other than that, it's really not an issue until you really sit down with your headphones and just play by yourself. And it has some excellent features for live use. The layering and mixing is very useful, and I LOVE being able to save my presets - I often need the same piano/pad combi, or layer a Rhodes and a piano sound, or even need access to EP 3 and 2 consecutively. It's very nice the way you can quickly access them at the touch of a single button, and mix the levels with dedicated faders. It's obvious (at least to me) from practical features like these that Roland intended the instrument for the regular live user. It's also a sturdy looking and feeling instrument, with very nice keyboard action.
All in all, I bought it for its excellent value for money. I thought about going for the 300 but was disappointed by the sustain of the piano sound and the feel of the keyboard (odd, since it's supposed to be a compact version of the keyboard of the 700). I even thought about going for the 700 with the SRX Rhodes board, but in the end those were SO much more expensive that it didn't feel like it'd be worth it. I'd rather buy an entirely new one for less money in four or five years. When buying this instrument, be aware of its limitations and consider if they're going to be a problem for you - if not, it's EXCELLENT value for money and I can recommend it to anyone with full confidence.