You have to love a bit of FM synthesis really ... though as for 'getting your head around it' I've always found that when editing a sound, what you may have had in mind as the result is far from what you end up with, the smallest changes affecting the sound greatly. This of course is part of the fun!
Describing the SY77 as a DX9 on steroids sums it up quite well really. The keybed is virtually effortless to play - a really nice feel actually, and perfect for fast playing. I prefer it to the Motif keyboards (even the new ones, up to and including the Montage). I'd say the closest feel to any other board I've owned is the keybed you get with the 73-note Korg M3/Radias, but that's probably quite subjective, and in my case once I'm in the zone on a synth I don't really notice the feel of a keybed any more, it's all about the noises.
Given the similarity of the FM architecture, I'd choose an SY77 over the DX any day of the week. I'm pretty sure it can do anything that a DX can do plus of course you have the AWM section, onboard FX (which are not bad considering) and sequencer to supplement things as well as the more powerful FM section (same number of operators but more algorithms and variations of source waveforms)
It's also got two modulation wheels (in addition to the pitchbend) which is something I'd like to see on more synths. One is neutral when centred (thus has positive and negative throw) and the other is the usual neutral-when-all-the-way down - it's surprisingly useful for creative performance purposes though it's 50/50 that you'll grab the wrong one when you forget there are two
Sound-wise, I like it a great deal. If I was an FM expert I suspect I'd love it like I love the SY85 - though it's less of a 'wall of sound' than the SY85 (hehe), it's a very different animal, has a certain something about it and does all the usual things you'd expect from a really good FM synth, plus a few you only get from an exceptional one (mmm... RCM and multiple feedback loops!).
I've still not spent as much time with it as I'm intending to, but I'm looking forward to creating some new sounds myself (and trying to recreate an amazing lead sound I had on the DX9 and haven't been able to get anything close to since) and using them in some new music. I can tell from the manual it's got a lot of depths to explore.
Build-wise it's not quite as tank-like as the DX7 but it's still a solid bit of gear. I suspect the weak points would be the screen/backlight and microswitches as both have suffered a little over the years on mine. I'm planning to replace the backlight on mine for sure - and may end up giving it some replacement switches as well.
Overall my feeling is that the presence of an SY77 pretty much renders the presence of a DX9 unnecessary.
(I've pondered trying to pick up an SY99, but I think that would be overkill given the other SYs I have floating around the place!)
Dave may well have more opinions all round of course ... and I'd be interested to hear them if he does