Let me chime in since I had the WK-3000 for over a year which is essentially same as the newer WK-3X00 keyboards. I also demoed the WK-8000 several times which is like the WK-3800 with 88 keys and stand.
My answer to you is that Casio makes very good value instruments, i.e. you get a lot for your money. Unfortunately, because the price is so low (comparatively speaking), there are many compromises. The keys on the WK-8000 are piano keys (i.e. are full vs. synth-style) but I do not think they are weighted. Consequently, the action is not close to a real piano or a good weighted instrument (such as Yamaha Clavinova). If you plan to use the instrument for general-purpose play, I would say it would be fine but if you are trying to use it as a piano substitute I would say think about it as you will not build strength in your fingers and generally form bad habits.
The polyphony is probably not a limitation for piano playing (you only have 10 fingers after all) but that limits the use if you are thinking about MIDI work and doing some elaborate arrangements.
To sum up, I think the WK series offer excellent bang for the buck but, alas, you get what you pay for. I would say for amateur playing it is often more then sufficient. A lot of on this board gig for living, so Casio WK would not cut it but for home play it is great.