Like most 4-op FM synths, it has a lot of trouble with low-end response. You just can't produce any growly sounds down low. While many basses sound OK, you can never really produce a good, ambient droning sound. The presets are fair to middling. There is a good variety with the only really obvious thing missing being synth pads (although this is very difficult to do with 4-op sine only FM with no effects). Particularly excellent are the bells and one or two of the basses.
MIDI implementation is sparse but adequate. You can select omni on/off, send/ receive channel, channel info on/off (for patch changes), sys info on/off (for sys-ex dumps). Yes, there are bulk dump commands and this offers the chance to get hold of lots of DX21 patches that are available in the public domain.
The synthesis engine is 4-operator, sine-only FM and the sound is all that that implies. You have a choice of 8 algorithms (operator/carrier combinations. If you don't know what I'm talking about then you should find a good book on FM synthesis - there are hundreds available) with feedback on operator 4 only. Each operator has its own EG but, for some bizarre reason (perhaps price), Yamaha chose to have only one LFO per voice which can be routed to operator output level, voice pitch and that's about all. I think that this is also a problem with the DX7. I'd really like to see one LFO per operator, one for pitch modulation and one for overall amplitude modulation, even if this meant hardwiring them. The other problem is that there is no fixed frequency option on operators. This causes problems when it comes to programming subtle pads (which can be done on DX7s and even TX81Zs) and when I discovered the feature on other machines I was in seventh heaven! You can set the rate and level scaling on each individual operator which is a real boon when it comes down to programming acoustic instrument emulations. Controller programming is excellent with velocity response (over MIDI) for each individual operator being controllable. Breath controller and hold/portamento pedals are also supported.
It is an 8 voice polyphonic, monotimbral synth. On the back panel, there are MIDI sockets (IN/OUT/THRU), Output and Headphone jacks, footswitch jack, breath controller input and adapter input (9V). The DX100 was designed as a strap on unit and hence there is the option of powering the unit using batteries.
I like my DX100 a lot. It has an important place in my synth setup and I would never part with it, despite my frustrations with its synthesis engine. It does some superb digital sounds (the hard-edged digital sound that was so prominent in the mid 1980s) and a few passable acoustic instrument emulations (the Sitar's an absolute killer and I programmed a reasonable pan-pipes patch). If you can get one cheaply then it's most worthwhile.