I feel i may be very well placed to answer this... having owned a bel canto S300i and having borrowed for extended periods the ref1000's
Class D was interesting when it came along, in that it offered a lot of power, a lot of resolution and a lot of clarity, for not a lot of money. In that respect class D does what it says on the tin. Fantastic grip (on the larger models) and a heck of a lot of punch, also on all models that ive heard, the midband texture and detailing are superb.
But, and heres the few exceptions that i found over long term ownership. the lower powered modules, like the 300w module in the S300i, struggle into tougher loads, the amp didnt enjoy trying to run my Dynaudio contours of the time. The ref1000's were faultless in this respect.
The other issue i had with the class D amps was their treble. I found them just a touch dry at the top end, a little too clinical and on occasion just a bit bright. There was a slightly glassy hardness to the highest frequencies via the class D that my focals just exposed brutally.
The ultimate test came recently, when my friend, the owner of the ref1000's moved to a brand new accuphase E-550 amp, which is a 30w class A design. In all ways it bettered the bel canto's. It was smoother, more dynamic and had a cleaner and sweeter top end that was far more listenable.
If you have smooth, warm slow sounding speakers, the highly damped and dry nature of the class D amps may work well, but on clean, fast and open speakers like my focals, the combination was not one i could live with myself.
They are worth hearing for their sheer clarity, and texture and IMHO the cheaper amps and integrateds offer better value for money than the big monoblocks.
Before their departure, i had a chance to hear the S300 power amp along side the ref1000 monoblocks, and in many ways the S300 was remarkably close, and in some ways, even slightly better, a tad richer in the mids, and a bit more enjoyable. Just lacked the drive and grunt of the bigger amps.
If your going to get into class D, try one of the cheaper lowe powered models, like the S300 and see how you get on