IMO, 25 keys is only useful for 1 handed basslines/triggering/leads (though even here you can run out of space), as a secondary keyboard or for a stripped down portable travel system. If you're looking to learn, one of the first things you'll learn is that you need more than 25 keys. 
 
The less common 37 keys is the absolute bare minimum for general purpose work, but 49 keys is a more practical minimum and if you have less than 49 it will severely limit 2 handed playing of any kind. 61 is better.

drewfx1
IMO, 25 keys is only useful for 1 handed basslines/triggering/leads (though even here you can run out of space), as a secondary keyboard or for a stripped down portable travel system. If you're looking to learn, one of the first things you'll learn is that you need more than 25 keys. 
 
The less common 37 keys is the absolute bare minimum for general purpose work, but 49 keys is a more practical minimum and if you have less than 49 it will severely limit 2 handed playing of any kind. 61 is better.

+1 Exactly.
 
I have the Axiom 25 for just that purpose. The knobs and pads on it are useful at times, and aftertouch is great for those instruments that support it. It's just a small, nice, controller keyboard for one-handed composition that you can keep near the computer desktop.
 
Just picked up a used a DGX-230(YPG-235) for $125 - that'll do for two hands when absolutely needed. (Way too damn big to be near the computer though...)