I have the SD STK S4M in a set of SD Stk-S4. It's used in a Stratocaster American Special Strat.
Tone:
The (S4m) tone is okay, but nothing too great. First off, the set is rather low output, making it a good choice for rock, where you use moderate amounts of gain. Keep in mind, that you will not have as much clarity and sparkle as when using actual single coils (even in split mode), playing clean.
On the other hand, for playing in higher gain regions, you have a rather fat bridge and neck pickup, which makes the in-between positions way less desirable. Although when split, the reverse polarity works as expected. You get a hum-cancelling, (slightly) humbuckeresque sound.
As the pickup works well with the set, I'd buy this pickup with the others, but probably not as a single mid-PU.