I had a D70 years ago and loved it. It was similar to the D50 but used the U20 waveforms. There are two card slots on the back and you can expand the D70 with U20 sample cards. There were even custom cards available with samples and programs if I remember correctly.

The samples weren't the best ever but the D70 has lovely filters and you can set the start and loop points of the samples giving you quite a bit of flexibility on the programming front. I was able to coax some great sounds from mine. As stated, the piano and string patches were very good.

What I particularly liked about the D70 is it had real character. It didn't feel like you were playing a bunch of electronics. It was quirky - in a nice way, a bit like a real instrument.

I used my D70 as a master keyboard for years and when it finally died I tried every keyboard in the music shop looking for a replacement. In the end I couldn't find any board from the usual suspects, Roland, Korg, Yamaha, Emu that had the same feel. Their sounds were better (it was about six or seven years on from the release of the D70 by then) but they seemed lifeless in comparison. I ended up buying a Kurzweil (at huge extra cost!) to feel I was buying an improvement.

There are many better keyboards on the market now of course, but you could do worse than buy a D70 if you're looking at second-hand boards.