I've been using a WK-1630 for the last few years as a composition tool, and it has served me well-- especially since I'm not a "real" keyboard player (I'm a guitar player). It is by no means a "professional" board, but works well enough for the hobbyist or semi-professional.
Mine was a reconditioned model I ordered through Musicians Friend, and was only $200 (normally $300). It has 76 keys, and plenty of different sounds (including the ever-popular "General MIDI" soundset) and a few decent-sounding drumsets, too. It also has a built-in sequencer, touch-sensitive keys, and some passable built-in DSP effects. It has a rudimentary synthesizer, too, so you can edit the preset patches. It also has a built-in 15 watt amplifier that drives four speakers, which produces a decent amount of volume for practicing.
There's only a couple major drawbacks for me: this beast is HEAVY, mainly due to the speakers, so it's not very convenient to bring to gigs and such if you're used to travelling light. Also, the only audio out is a stereo headphone jack, so it's not particularly well suited to recording or running through a PA. However, I have run it through a small PA before and have also recorded it (for demos), and it's not half bad.
The newer Casios in the WK series (the WK 3000 and WK 3500) have many more features, plus SmartMedia drives, and are around $300-$350 from most major dealers. It's a lot of bang for the buck, IMHO.
Here's some reviews.