If the question is whether or not to get a Korg Z1, just get it.
You'll love it. I've been doing synths a long time and the Korg Z1
is hands down the best designed synth I've ever had the pleasure to
play. It's got so many special wonderful features that it really set
the standard for great hardware synth design. Since it's day, much
of the action has moved to software. You can find details on this
forum and around the net. There are legends about the instrument's
origin and design.

But it's not perfect, and while it has many great features as a MIDI
controller, including those mentioned in other responses to your
post, it does have one weakness in my opinion. The keyboard is well-
built and has a good feel (for an unweighted keyboard), but it's
difficult to control all the velocity levels. The lower (soft)
velocity levels are particularly hard to control. If you're into
dynamnic nuance in your playing, as I am, you might find the Z1
keyboard a little frustrating.

If you like to mess with the sounds, the Z1 excels. There are lead
sounds on the Z1 that I love that I can't replicate anywhere else --
though I wish I could. But I usually play my Z1's (I have three of
them -- used to have four) from another MIDI keyboard. If I want
full-weighted piano-type action, I use my Kawai MP9500. If I want
lightweight synth action, I use my Kurzweil K2661. I can play lead
sounds on the Z1 much better from the K2661 keyboard than I can from
the Z1's own keyboard. I can easily get dynamic nuance out of the Z1
playing the K2661 keyboard, which means that the Z1 can respond very
sensitively to MIDI. The problem is the internal MIDI output of the
Z1's keyboard. I've tried every setting possible on the Z1 to try to
remedy this, and you can make it a little better, but nothing really
helps too much.

Still, even with this limitation, I do play fairly frequently from
the Z1's own keyboard also. It's quite playable as long as you're
not trying to play a lot of dynamically nuanced passages at soft
dynamic levels. The reason for playing from the Z1's keyboard is so
that you have immediate access to all the other wonderful controllers
on the Z1, especially the X-Y pad which is nothing short of a miracle
and makes the synth indispensible by itself, even if weren't for all
the other great things about the mighty Zee!