I upgraded from the Model: Cycles, which I really liked, to Syntakt. You get 12 tracks instead of 6 and two LFOs per track. The sound design machines seem a bit more limited but there are more of them so, overall, you probably have more sound design options. I've not owned a Digitone or Analog Rytm but it seems like you've got both combined in one box. There is no sampling, obviously, and you are limited to a specific set of FM-type synth sounds - although the analogue dual VCO can create a surprising range of sounds because of the multitude of waveform combinations available: from Moog-style bass to disruptive noise. And the analogue drums are great. I've only had Syntakt a week but I can see that it's going to get a lot of use. With a bit of imagination you could almost certainly do a live set with the Syntakt alone. It's easy to create generic house, techno etc but if you experiment with varying track lengths and probability settings you can go beyond rigid, grid-like dance music. It works really well for ambient and generative music, for example. As with all Elektron products, the manual is long and there is a steep learning curve - but if you are familiar with their sequencers from other products then that will speed things up a bit. For me, the advantage to Elektron products is their portability: they get you away from the DAW and allow you to make music anywhere - and with 12 tracks that really can mean complete songs. Mine will be going on holiday with me this summer.
Edit: I've had this for about three months now and it's probably the best piece of gear I have ever bought. I own a modular synth and loads of other stuff but this is getting far more use at the moment. The range of sounds you can get from it is really surprising: Aphex- style IDM, cold wave, chip tune, dark ambient - even gamelan-style percussion. It's a complete analogue and digital drum machine AND analogue and digital synthesiser. Yes, there are limits to it - as with all gear - but there are usually workarounds. The only thing it doesn't really do is lush synth pads - so I bought the Digitone for that. The Digitone has more depth for pad sound design but it's difficult to put together complete songs with only four tracks, even with sound locks etc. But together they make a pretty fantastic combination. In fact, Elektron gear is so good that I'm now thinking of getting the Digitakt in the distant future when I can afford it. I'm definitely a convert.