Tom,
I have the 73 key version with Radias expansion card. The expansion card integrates perfectly in the sequencer, but has the disadvantage that it uses the M3's effect section (shared with other tracks in the setup).
As a result, some Radias presets may sound weak in a multitembral setup. However, using the track sampling option, you can overcome this issue.
About the karma-thing:
Karma is a lot more than an appregiator since it can drive any parameter meaning you can use karma to change soundscapes as you play. On top of that, Karma can generate pieces of music on the fly, the way Karma composes these pieces can be driven by changing paramters. I try to explain: a simple drum pattern can be tweaked by a knob resulting in a far more complex drum track. Mind, sometimes, you get unpredictable results.
Also, you have the ability to create your own Karma GE's, meaning, you can create about any preset you like. Combining this in a multi tembral setup and you have about anything you can imagine.
The M3 is really a groundbreaking device, but I have to warn you. It's far from a conventional instrument and you have to understand how this this is working to get the best out of it.
What I particular like about this device is the optical SPDIF in and out, which alow you to integrate computer based setups (letting you to combine VSTi's)...
As for the sound, you have the M50 so add the sampling function combined with the radias.... need I say more ?
Perhaps some words of critism: for lead sounds, I keep looking my sounds outside the M3. Here, my Virus TI comes in really good. The sound of the Virus really complements the M3, so the M3 probably won't remain or become your only synth...
Also, the way it is build: it has a great design, but somehow the device doesn't looks so solid like yamaha's or Rolands counterparts. If you use the synth in a studio, than this is not an issue. For usage on stage, I'm not sure how long the M3 will hold in comparison with others...
To sum it up: The M3 is a great instrument that I can recommend without any hesitation. It's really a great device to use in your studio.
I hope this helps...
Moon