I purchased 2 of these to act as sostenuto and sustain pedals for the M-Audio Keystation Pro 88 and they do the job and work with a variety of devices - I've since replaced my M-Audio keyboard and replaced it with a Korg digital piano and my remaining SP1 performs admirably (I say remaining as my Korg only has one SP input).
What to get with the SP-1 is a basic package - a plastic, non-latching pedal with a rubberised base that basically switches from off to on (or on to off if you use the polarity reversal switch on the back - this is essential for some devices in order to get it to work in the traditional manner).
Years later, I'm still using this pedal for sustain but the sheet of rubber on the bottom has started to peel away and is difficult to get back on (it has little rubber pins that slot into the base). It has also developed a distracting squeak but I would expect that after years of use - that's nothing a bit of WD40 wouldn't solve!
My experience with the SP-1 is that it is an adequate pedal, gets the job done and, if in a pinch, I'd get one again. However, long-term I'd probably be inclined to get a more traditional single-unit setup instead of 2 separate pedals.