What exactly is the MX88?
Thanks for the question G. And although "just" is not a word you'll often find in product descriptions, we'll attempt to put the MX88 in perspective.
Yes, it has 88 weighted keyaction, Yamaha's Graded Hammer action.
The MX is the entry level of Yamaha synthesizers - meaning you can edit and get at deeper functions (if you are so inclined, more on that later). The current Yamaha synth line includes the MX Series, the MOXF Series, the S-Series, and the Montage Series. (Motif-Rack XS is a current Rack module; The MOXF includes a built-in Song and Pattern Sequencer; the S-series is the sister product on the same level as the Motif series without the Integrated Sampling Sequencer; Montage is the new flagship synth).

Bridging the Gap Between Hardware and Software
"The new MX series combines Motif XS sounds, USB Audio/MIDI connectivity and advanced DAW/VST controller features at a ground breaking level of affordability. - Over 1000 sounds from the MOTIF series - Equipped with VCM effects - Easy Split /Layer Performance Mode for Instant Inspiration - 16-Part Multi timbral and 128-note polyphony - Extensive Hands-On controls for VSTs and DAWs - Bi-directional USB Audio/MID interfacing - A complete suite of music production software included - A lightweight design with a quality Yamaha keyboard - Complete analog and digital connectivity."

Bang for the buck is off the chart... it acts as it's own MIDI and audio interface, all you need to record to a laptop or computer is a USB cable and the appropriate driver for your computer type; it is also iOS compatible (requires a USB-to-Lightning adapter); it can act as a remote control surface for Cubase, Logic Pro, Sonar, and Digital Performer, it can act as a direct hardware controller for your VST soft-synth plugins, the AI Knob (Data Dial) acts as an immediate assigned controller in the Cubase DAW - just hover the mouse over the VSTi parameter and you're controlling it (makes controlling softsynth plugins a breeze). A free download of the current Cubase AI comes with it.

At only 32lbs. the MX88 can easily be handled without the help of other band members to setup.
The more than 1000 sounds from the Motif series means you are playing the same level sounds that dominate the stage for the last 15 years (the Motif era)... if you are looking for a solid synth that covers a wide palette of sounds, with a killer piano sound and feel, that can be the instrument centerpiece of your home recording rig.

And just to top it off, you can combine it with an FM synth via the free FM ESSENTIAL App, that when connected to the MX joins with it to be a controllable dual synth, sound combines with the MX so both the FM plus MX sound is Output via the MX's main L/R outputs. Sweet! You can edit the FM synth using its front panel Knob section. In addition to unlocking the FM ESSENTIAL iPhone/iPad app, the MX88 will unlock the CubasisLE2 app for iPad.

All in all, it should be clear how this is completely different from the Yamaha Stage Piano (CP4/CP40 Stage) and completely different from the DGX660 which is more a family room, accompaniment based instrument. DGX is both GM/XG compatible and features score and lyrics. And is designed to be assembled and is not the easiest thing to move to gigs. And while Stage pianos and accompaniment instruments, like DGX, have some similarities with a "music synthesizer", they clearly are not as focused as the MX Music Synthesizers are on being inline with what the Motif, et al, do... elegant computer connectivity, and more depth in being able to edit and customize the sound itself, MX is designed with a wide palette of excellent sounds, as well as the portability for gigging musicians.

With an iPhone, iPad, Laptop, or Windows PC or Mac computer, the MX can give the musician on a budget, a no-compromise sound/feature set that enables them to build a home recording studio that can be easily transported to the gig. And just go the extra mile, if you do get curious, it has the same 8 Element architecture and VCM Effects that made the Motif XS/XF the successes they are... So there is a path to deeper levels of editing...

It has the huge Motif Library, but you can also make multi Part Performances that combine Parts as Split or Layered sounds, there are two Arpeggiators, and additionally a third (Drum) Rhythm Part can be added... all in all the MX88 is hard to beat.

Thanks for the great reply!

So.. what am I getting "extra" if I choose the MOXF 8 instead of the MX88...?

You can do the specification comparison at the website... USA.yamaha.com

The MOXF includes a built-in 16-Track Song and Pattern Sequencer.
It has an A/D Input for external devices from Microphone level through to Line level for a mono or stereo input.
It has dual Stereo audio outputs via USB (4CH) 4-in/2-out audio interface for Windows/Mac
A built-in Vocoder...
Flash ROM Waveform expansion - compatible with Motif XF Level sounds - you can add a gigabyte of custom XF Waveform data extending the full time Wave library by 2048
VST3 Editor allows advanced audio routing integration with Cubase AI
Bigger screen, more on-Board programmability
Entire 3,977 Motif XF wave ROM (741MB)
Expanded Articulation Voice architecture
Arpeggio engine x 4 (Motif type)
Motif type performances (4 Voices) with 6 sets of arps for each
9 x 128 preset Voice banks (complete Motif XF Voice set)
Arpeggio Library 7,891 Arps

Bad Mister wrote:

You can do the specification comparison at the website... USA.yamaha.com

The MOXF includes a built-in 16-Track Song and Pattern Sequencer.
It has an A/D Input for external devices from Microphone level through to Line level for a mono or stereo input.
It has dual Stereo audio outputs via USB (4CH) 4-in/2-out audio interface for Windows/Mac
A built-in Vocoder...
Flash ROM Waveform expansion - compatible with Motif XF Level sounds - you can add a gigabyte of custom XF Waveform data extending the full time Wave library by 2048
VST3 Editor allows advanced audio routing integration with Cubase AI
Bigger screen, more on-Board programmability
Entire 3,977 Motif XF wave ROM (741MB)
Expanded Articulation Voice architecture
Arpeggio engine x 4 (Motif type)
Motif type performances (4 Voices) with 6 sets of arps for each
9 x 128 preset Voice banks (complete Motif XF Voice set)
Arpeggio Library 7,891 Arps

Thanks again!