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.
Update, almost four years later: I'm still using Syntakt a lot and I would still say it's the best synth I have bought. But if you're thinking of buying one it's important to consider two things: firstly, the limitations of the device (and whether it suits you) and what it offers compared to other Elektron devices in this range. These days, I also own Digitone II and Digitakt II and they all have unique strengths. Digitakt II is easy to define – it's a sampler. And Syntakt is not. Having said that, it's rumoured that Syntakt will soon have a basic one-shot sample playback capability, so it's worth bearing this in mind. Digitone II is a great polyphonic synthesiser that excels in making complex, experimental sounds; the drum sounds are pretty decent but they're not its best feature – they can sometimes be a bit harsh, as is often the case with digital percussion. Overall, Digitone II is a brilliant all-round groovebox that has huge sound design possibilities – and polyphony.
Syntakt, however, excels at drums – particularly the analogue kind. In fact, it's perfect for hissy, analogue techno. It's ideal for coldwave, industrial, EBM etc. If you want to sound like Cabaret Voltaire, Suicide, Aphex Twin and so on, then this is the Elektron box for you. In fact, even the digital drums can sound great with a bit of tweaking and use of the onboard overdrive. Its detractors say it's just a pared-down Analog Rytm – to which I say: fine! It costs less. But it does have more voices to play with and Elektron keep updating it with new machines – plus it's smaller and more portable. The downside is that (like Analog Rytm) its synth machines are monophonic – aside from the chord machine, which has its limits. So you're always going to struggle if you want pads. This is worth keeping in mind. If you have the money, you could always pair it with Digitone II for the best of both worlds.
What I'm saying is that Syntakt has a particular sound which lots of people love and some don't. When people criticise it, saying: “it doesn't sound good” then you only have to look on Youtube to see that's not the case – you can do fantastic things with it. The issue is more whether a monophonic, semi-analogue synthesiser is the best device for your music. If you want to make commercial house or chart-friendly pop music, then it probably isn't. If you want to channel Throbbing Gristle circa 20 Jazz Funk Greats or Aphex Twin doing Analord, then this is most definitely the groovebox for you. Final thought: it's been a few years now, so could Elektron be readying a Syntakt II for release? If so, it'll probably halve the second hand value of the current Syntakts – worth considering if you're going to buy one...