For the price, Keystudio with Sessions is a great value. You get a 4 octave keyboard plus the music composition software, which is like a PC version of the Mac's GarageBand. My two main gripes however are that there is no Midi Out port on the keyboard. So, if you've got external Midi gear, you'll have to plunk down some extra money to purchase an external Midi interface. Also, my computer is a little underpowered so I had many latency issues when running Sessions and using the real-time effects. Lastly, Sessions does not let you edit the Midi information note for note, although you can quantize your track.
On the good side, the keyboard is sturdy and is powered through the USB cable. The software works very well, with an excellent range of instruments, loops and effects. Just make certain that you're running at least a 1.6 ghz P4 with 1 gig of memory. I had less memory when I purchased my keyboard but the added memory helped the lantency problems a bit. I have a 1.2 ghz P4 which also makes Sessions less responsive, though adjusting the latency slider has helped enormously. Lasly, Sessions has just been updated to version 1.5 and while I haven't really had a chance to play around with the update, it promises to improve responce time. I really wish M-Audio would add the ability to edit Midi information to the software since this would really make Sessions a simple one-stop shop for getting your musical ideas into your computer.