Great sounds, great size
This is a terrific performance keyboard! It's small and light enough to schlep around to gigs, but still provides both great sounds and a usable master keyboard. The programming interface is a little off the beaten path, but is quite usable if you plan ahead. The MIDI control is better than some dedicated master keyboards. I've been looking for a second keyboard to cover a lot of bases in classic rock band, and this one does the job nicely.
One a 0-10 scale, using several $2-3K keyboards that I own as reference ...
Features: 7.5 -- great sounds and a rugged, semi-weighted keyboard in a small footprint for under $1200 leaves the SP4-7 with little or no real competition. Full-size Pitch and Mod Wheels are nice to have. The display is small, but very readable, even on a stage. This 76-key product is smaller than some 61-key competitors, yet I don't miss the extra 12 notes of a full 88. Splits and layers are easy to make. I wish it had a few more controls for live use.
Sound: 8 -- great acoustic pianos, brass, and strings. Very good Rhodes, decent Wurli, fair Clavinet and Hammond Organ. Not a lot of synth sounds, but there are some useful ones, and you can layer them to make more complicated lead patches. If you know someone with a PC3 or PC3LE, they could make more sounds for you. The only reason that it doesn't get a '9' or '10' is the library is kind of small (so far, anyway).
MIDI Control: 9 -- four zones, which can operate locally, or send separate channels of MIDI to external modules. Each zone can react differently to pedals, the Mod Wheel, and the control knob. The only thing that would make it better would be 4-5 knobs; the round-robin selector button isn't all that live-performance friendly.
Programming: 5.5 -- You can get what you need out of this keyboard, but the programming interface is far from intuitive. Also, there aren't many editable parameters: You can change the delay level, for example, but if you want a different delay *time*, it probably requires a new patch -- which can't be generated from the SP4-7. In addition, while each patch has five adjustable parameters (timbre, modulation, envelope, effect, and reverb), most of the current patches only provide 1-2 adjustable items. If you have access to another Kurzweil keyboard from the SP3 family (and the newest OS), you can build patches on the other keyboard and download them to the SP4-7. For the rest of us, we're stuck with the onboard patches, and 64 more that are on Kurzweil's website.
Reliability: 8.5 -- I haven't had it that long, but the chassis is very solid and the keys feel substantive. Not a lot of things sticking up means not a lot to break on the gig.