Manual: http://www2.yamaha.co.jp/manual/pdf/emi/english/port/PSR90E.pdf

- What type of keyboard (synth, piano...)? How many octaves?
This is a 5 octave electronic keyboard, not a synth nor a piano. Not even close.
The touch is like most synths: ok, but no dynamic touch or after touch here. This is an extremely basic machine with crap sounds and it was probably extremely overpriced when it came out around 1982. However, if you stick it through some good effects, i'm sure you can use it quite adequately.

- What connection types are there? (audio, MIDI...)?
RCA stereo out, some sort of effects loops out & in (still stereo RCA) but i've never used that...MIDI in & out...exp pedal and footswith. Power is a separate adaptor.
The keyboard comes with a plastic & metal stand that fits on it and lets you have sheet music to read while you're playing, as seen on the photo here. You can take this off.

- How many different sounds and/or styles available? Can they be edited?
Not a lot. Check the manual but we're talking about 24-36 here.
You can't edit the sounds other than add chorus, reverb, portamento.

The rhythms are the only reason one would want to buy this, along with the fact that this is a very simple drum machine with some fairly decent sounds, surprisingly enough.
The rhythms are cheesy & fun, it's great, you can have a looot of fun with this. Plug it into an expander (i used my Roland GR-30 MIDI guitar expander with no probs) and you can actually have a good time, but i'm not sure you can do much with this thing apart from that.

- Is there an effects section?
Nope

- What are the features/characteristics of the sequencer?
4 tracks, you can save to cartridge if you find any on the second hand market (good luck) or to cassette or minidisc. Those of you who have had a Sinclair ZX Spectrum or a Commodore 64 will know what it's like.

- Is the general configuration/setup simple?
Oh yeah!

- Is the sound or effects editing easy?
The what?

- Is the manual clear and sufficient?...
Yep.

-monophonic (one note at a time) analog synthesizer.
-six octave steps.
-Audio out right and left (mono), ext in, filter audio in, Midi (in,thru,out), two standard male DB-25 connector.
-Bank with 128 presets, entire banks or individual presets may be uploaded and downloaded via MIDI (through Mac/PC).

When you know how to use synthesis, you take the moog immediately in hand. Otherwise some hours are needed, but the textbook (in English) is very well made!

I compose electro-house music, I do not think that there's a better synth for this music type. Sounds make me cry. I have a virus which however is a good synth but there frankly, the moog surpasses it ! There's only filters effects, which are very goods (moog filters...)

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Midi in, out, thru, headphones out, 6 separates mono outputs (from A to F, A+B can be used as stereo main out) and two in that can be used as trig.

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You have about 9 synths (GND, TRX, EFM, E12, P-I, MID, INP, CTR, ROM and RAM) divided in machines. Machines can be bassdrum, snare, audio in, external/internal midi controller... tons of stuff.

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Elektron made C6 for sysex and samples transfert. You can dump all your kits and patterns, or juste some of them. And for the UW version, you can sand/receive samples in the rom section.
Some people created editors made to work in a musical way.

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Each track have his own FX page, with a two-band filter, a sample rate reducer, a eq thing and a modulator fx that gives something close to ring modulation. Via the routing page, you can send each track to reverb and delay included in the MasterFX wich also contain a compressor and a multi-band eq. But the MasterFX only works in main out mode.

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It's a Tr-like with avanced features : a swing for groovy patterns, the setting is global but you can set the swing steps for each track. Same thing for the accent. Also a mute mode.
Sequencing md is all about the parameter lock. It makes you able to set different machine settings for any step of any track... it is simple to use, and powerfull. There is a slide function that you can activate for any step for all track at once or for each track.
And sound become powerfull and versatile in way you often can't imagine. Seriously.

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Oh yeah. This machine is made for live and on the fly work. You can edit everything and hear the result in real time. For each track you got three pages : synth, wich is specific for each machine, efx page and routing page wich is the same for all track. In routing page, you can set the amount of disto, volume, pan, reverb/delay send, lfo speed/depth/mix.
16 lfo, one per track that can be routed the way you like (yes, you can rout 16 lfo to one track), set the speed, the depth, choode one of the 6 waveforms + the same waves reversed. For each lfo you can use two waveforms at once with the mix function, working like a crossfader.

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Yep. Made for various kind of electronic music. With the sampler... why note make drum'n bass? Nothing is impossible.

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Nope, but you can use samples, so...

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Good, pretty good. Simple and effective.

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There is no velocity, but you can play with volume setting and accent.
The sequencer is truly binary. I mean, you dont have that groove feeling, like in 909 or 808. It would be sooo cool.