Friend of mine recently switched from the 1k to the 2000XL, and now he's trying to sell the XL to either buy the 1k again or preferably the 2500.
The main reason is the lack of effects, transportablility (if that word exists) and the fact that you cannot create a mono program (so that each pad stops the previous pad).
The lack of effects really makes a difference, unless you like to load your samples first into a computer and add effects to them there or use some other method. Or buy the effects board.
The XL is also slightly slower + the filter and general operation needs getting used to when switching from the 1k . On the other hand it is more sturdy and just looks so much better than the 1k.
