However, if you are only keen on the piano sounds and do not want to have the other features such as styles and effects, then you should have a look at the models in the P series as well.
The keyboard here is a graded one where, just as on a real one, the keys are heavier on the lower octaves and much lighter on the higher keys; perfect for playing the piano. Yamaha calls it their Graded Hammer Standard or GHS action, which is among the best they have.
You also have the option of connecting a three-foot-pedal unit to your keyboard, in case you want to have the same range of expressive effects as on a real acoustic piano.
In terms of voices, it's not just the pianos and electric pianos that you get, you also get some cool organ sounds, strings, brass and lot more, including several Live! Cool! and Sweet! Voices. The best part is that you can make these even more appealing using the several effects; you have a collection of DSP effects, ambiance effects, distortion that you can apply to the various voices.
If you're learning to play the piano, you can make use of the Yamaha Education Suite (Y.E.S.) that can further supplement your learning. Even the screen can display the music notations when you play back a song. If the song contains both lyrics and chords, both will be shown. Experienced players however, are less likely to use these features.
It looks sturdy but not as beautiful as a piano though; the "wood" portions are made up of synthetic material.
The Yamaha DGX640 keyboard is highly recommended if you are looking for a portable piano with excellent key touch.