In my experience Quest B6-4s are equal to or slightly better performing than the Estes ones and have a real four second delay (or a bit longer). They are pretty much interchangeable in application with the Estes equivalents. In a B-impulse competition, I'll go Quest. Otherwise, whatever I have.
The A6s and C6s are NOT freely interchangeable with their near Estes equivalents.
The C6 has a good initial kick but then a long, lower thrust "cruise" burn. This makes for spectacular flights in light rockets and big arc-overs in heavier ones or on very stable ones on windy days. I would not fly a Metalizer on one, for example. For something light and low drag they are really impressive.
The A6-4s are a disappointment to me. Others have said they're perfect in something like an Alpha but they build thrust so slowly that if there's much of a breeze even a light rocket will pitch over. Their time-thrust curve is triangular. But in a cluster - nice!
And yes, they're smoky. They are also all noticeably louder than their Estes counterparts - which has a certain appeal to some

.
I occasionally buy bulk packages of Quest motors when they go on sale, and then I reserve the supplied Q2G2 igniters for clusters (and drag races). A slightly melted Estes B6 plug (the purple/pink one) and an Estes igniter work fine in the Quest motors, so I can then have the marvelous Q2G2s available for special purposes. Every cluster flight I have flown as a BAR has been with Q2G2s (including two today - using 3x 1/2A3-4Ts) and every one has lit all the motors.
As always, YMMV.