Was going to post this yesterday and ran out of time - some points already covered by other posters, but you may find something of interest...
I've used various Soundfield mics since the early nineties to record loads of different sources: vocals, acoustic guitar/bouzouki/mandola, uilleann pipes (the best mic for this I've heard) string quartet, piano, drums and percussion (great on djembe), double bass, sop./alto/tenor saxophone, choir, centre pair for an orchestral array - you name it.
Upsides: breathtaking neutral-but-warm sound (how do they do that?), at least stereo ability - surround with the right model. Buy the less expensive SPS422B which now has B-format outputs and you have surround for the price of stereo as you can record B-format to four tracks and decode any way you like with the Surround Zone plug-in (costs a fraction of the hardware decoder, - check the website for platforms). Excellent vocal (and speech) mic - very good for calming a ssssibilant ssssinger now, though the older Mk 4 always sounded a bit bright to me.
Downside: the neutral nature of the sound - you may prefer something more coloured on occasion. Wasn't that keen as drum overheads - in the rooms I tried more conventional mics worked better, but drum *ambience*, a different story. Very good on grand piano, but whenever I had the chance to shoot out against B&K omnis I preferred the B&Ks tonally. Not a cheap mic, but SF is like having an entire mic locker in one unit.