Very popular, actually. Conns were the horn of choice for many a fine player. In fact, one model is known today as the "Chu Berry model".
Conns had a great sound, big and full. It wasn't until Selmer introduced the Balanced Action, followed relatively quickly by the (rightly) legendary Mark VI, that Selmer began to pull away from the pack. This was due mainly to Selmer's design re: key layout that facilitated rapid fingering, as well as the refinements they made to the horn's basic intonation and evenness of sound.
Those old horns (King as well as Conn, and even Buescher, which was Ike Quebec's horn of choice) often need a little extra "attention" when playing them, and the key layout at first seems ridiculously spread out and unnatural), but the attention is rewarded by a sound that is simply beautiful. I had a chance to play an old Selmer "cigar cutter" once, about 25 years ago, and althought the intonation was hairy, to put it mildly, the tone was to die for.Which was what I was hoping the owner would do...