Pretty Good Compromise
I needed to replace a very cheap lightweight keyboard (Yamaha ESR-P223) on which a middle-range oft-used key had failed after three years of heavy use (40 or 50 gigs last year). My main need was for a portable keyboard for outdoor gigs where no AC is available, hence a self-contained battery-opp model was called for. I also wanted something light enough to carry on public transit. No, I'm not kidding.
The NP-31 isn't too heavy (just under 13 lbs, according to the spec sheet), but it's heavier than I was anticipating, and because it's longer than what I was accustomed to (76 keys vs 61), it can be a bit unwieldly on the bus. My bandmates say the built-in speakers sound good to them, but they seem a bit tinny to my ear. Also, if it's turned up to near-full volume (which is necessary at these unplugged gigs) the thing eats a fresh set of batteries (6 AAs) in less than 4 hours, although the specs say the batteries should last up to 6. The action is good, allowing for much more expressive playing than the lighter organ-style keys on my previous model.
A digitial display would be nice: something to indicate which voice/sound you've selected, whether you've transposed and how far, metronome speed, and other useful data. As designed, you have to remember all of these things.
I spent almost $100 more than I was hoping to, but I did get a superior instrument to the portable one I was replacing. The NP-31 is also a good alternative to my other stage piano, a very heavy, bulky Ensonic KS-32 weighted-action model I may not be taking out much at all anymore.