Great combination!
So you're considering to buy a used XP-30? I have bought one new in 1999 or 2000 and still use it, now as a second keyboard above my Juno Stage (with SRX-07 board). I think it's a great combination and it really works for me! Also my fellow band members love it. I have the "Keyboards of '60's & '70's-" expansion board because I found it essential for organs and EP's and the Pop expansion because I had it lying around (in my JV-880 which I rarely use). But also without these 2 expansions, you have all sounds of the JV2080 + the Session, Orchestral and Techno Collection SR-JV80 boards. A great collection of patches (over 1400 patches, more than the Juno Stage has). I use this combo now (JS + XP-30) for over 4 years and I can't think of a patch that I don't have or can't create with these two stage 'friends'. I even used the XP-30 for about 5 years as the only source of sounds, having an A-33 midi keyboard (76 keys) to play most of the sounds and the XP-30's keyboard as second keyboard on top. Nowadays I use the Juno Stage instead of the A-33, to have more possibilities, be more flexible and have some fresher sounds (and the A-33 had some broken keys).
Soundquality
As I said, you have all internal patches of the acclaimed JV2080. That's a great start, but it gets better. Because the session-board is onboard, you have a great grand piano (not as vivid and expressive as the 88 stage grand in the JS, but still good enough for any gig in my opinion), acoustic instruments like flute, trumpet, and a wonderful nylon guitar and some nice synth-patches.
The Orchestral board contains some very nice string variations, solo instruments and ensembles as well as typical orchestral percussion.
The Techno Collection has some creative electronic sounds, but nowadays I find them a little outdated. There are some very useful waveforms on that board, though.
Besides the headphones output, the board has only two analogue outputs, just like the Juno Stage, enough for stage use.
Some of the waveforms are (exactly) the same in the XP-30 and the Juno Stage, so you could say they sound alike, still the Juno Stage has a great deal of better waveforms over the XP-30 (acoustic pianos, EP's, acoustic guitars, bass, drums).
Structure
You have basically the same sound structure as in the Juno Stage: 16 parts multitimbral performances, patches contain 4 tones (no 'stereo'-tones like JS), MFX (only 1, 40 different effects), reverb and chorus. You can create any splits and/or layers spread over the whole keyboard. You will find almost the same parameters for editing tones, patches and performances (WG, TVA, TVF, LFO, FX), although the Juno Stage has a few more, especially in the performances.
Controls
The keybed has almost the same response as the one in the Juno Stage (semi-weighted synth action) and I find it really expressive. What I like most about the XP-30 over my Juno Stage is the 4 sliders. These sliders can be used for either tone level (to control the level of each tone in a patch), attack/decay/cutoff/resonance (similar to JS control knobs), or they can be assigned to any control you like (which I find great and is a function the JS lacks). Also it has (channel) aftertouch (not on JS!) which I like to use a lot on certain sounds (synth lead, brass). Of course you can also add an expression pedal which can be assigned to almost anything you want.
It has an extensive arpeggiator (derived from XP-60) which has user programmable presets and can be very handy and easy to use.
If I still have not convinced you, please have a look here:
http://www.soundonsound.com/sos/aug99/a ... ndxp30.htm
I would not use it as an 'only'-keyboard, not even on smaller gigs, because I really need the 76 keys for piano sounds, but that's my opinion. I would use the Juno Stage if I had to choose one.