Hana, I started out with the PSS-A50 some two years ago. My focus was on (double) bass so initially I bought it just to find out notes by ear to then transfer to the bass. For what it is, and for the price, it is a sweet little thing. It is super light and portable, has the neatest mini-key action I touched so far, and does well as a little MIDI controller too. The sound (through its single speaker as well as the stereo headphone output) has a little noise to it. The internal sound bank is so-so, useable for some noodling but nothing to take too seriously.
In my case, after just weeks I got hooked on piano VSTis, so bought a Roland A49 controller, and a few weeks after that, went on the hunt for a proper 88-key weighted hammer action. That resulted in the Roland FP-10. I kept the PSS-A50 for some MIDI testing/score writing/note entering and such.
Now CyberGene mentions the Casio CT-S1. I recently bought that too, for use as a second 'manual' above my 88 keys to create a setup for Hammond-ish playing (also through a virtual instrument). Again for what it is, I like it a lot. It looks stylish/classy, feels sturdy enough, has a super simple non-distracting layout, doesn't weigh much. So weight-wise it is as portable as it gets, size-wise this depends on how and where you want to take it. The keyboard action is nice, feels good to the touch, has the right amount of texture on the keys. Of course it is non-weighted, a springy action returns the keys. MIDI behavior is nice enough. Now, for such a keyboard, I'd say the sound quality is more than decent. The experience through its speakers is quite good, headphone output is excellent. Many of the sounds are pleasant and actually usable and enjoyable. My keyboard experience is not extensive, but thus far I've always been put off (or even appalled) by their sound and speaker quality.
So, here are some hands-on experiences, maybe these'll help you a bit.
Cheers and a happy search,