If you like the way the RM1x works and want a keyboard that's similar you should look into one of the Motif family since the sequencer is essentially a superset...patterns, chains, songs, etc....and the synth architecture is the same (with 4 elements
instead of 2). I bought a Motif ES after intially looking at the Karma, partly because I thought it sounded much
better (YMMV) and could sample.
I couldn't quite get my head around how Karma worked, it is sort of like an arpeggiator with artificial intelligence
but that doesn't really completely explain it. You can generate some amazing stuff with it but I just didn't understand how to control it.
The Motif arpeggiator is very sophisticated and unlike the RM1x you can actually take your own sequences and convert them into arpeggio patterns. They needn't be notes, arps can contain just controller data. While in the end Karma is probably more flexible (esp. if you buy the software), the Yamaha arpeggiators are easier to understand and capable of some phenomenal stuff.
I listened to Hatfield and the North at Rainbow. They were very wonderful and they made my heart a prisoner.