The timing of this post is interesting to me: I just bought a 1966 Slingerland kit last night. I paid half what an equivalent Ludwig would cost, and a third of what a Gretsch would cost.
Slingerland suffered from years of being shuffled from owner to owner. Each successive owner refused to invest in the company, even to the point of not upgrading the tooling machines. A major blow was when Slingerland stopped providing endorsement deals. As an example, Danny Seraphine had endorsed Slingerland for over 10 years, when (according to him) they dropped him for no reason. (Seraphine moved to Yamaha then to DW.) Phil Ehart (Kansas) was another artist dropped by Slingerland.
When I was a jr high drummer (late 1970s) I had a Ludwig Hollywood kit, but I LOVED Chicago and Kansas. And of course, I wanted a Slingerland kit, so I could be like my two idols. But jump forward into the late 80s 90s, 2ks... what drum sets did we see on stage? What brands did the middle school and jr high drummers see?
Slingerland pretty much died in the 80s. Oh, it was around for years to come, but the brand was really just an also-ran. The worst time was the Music Yo era, when a Sears Roebuck set had a better tone than the particle-board garbage Gibson sold under the Slingerland name. Gibson let the brand die.
I agree with
@opentune that Slingerland drums have a wonderful, warm/fat tone, but in my opinion they have a limited tuning range. But that's not a bad thing: I've owned 5 Slingerland sets over the years and I've learned to find each drum's happy place. All the sets were 3 ply, I never owned the 5 ply. For me, I love the 13-16-22 combination and in these sizes, Slingerland drums do well for classic rock and blues. If I want to do big band, I'll switch over to a 12/16/20 set (which is what I bought last night.)
For hardware, I don't use vintage hardware with any of my sets. I use DW stands and pedals, and either Gauger RIMS or INDe mounts. I use Evans G12 coated batter heads, G1 clear reso heads. These drums seem to thrive with single-ply heads, and I don't want them to pretend to be a modern set.
To be honest, my Gretsch sets are my favorites. But I don't gig and if I did, I'm not sure I would risk taking the Gretsch out of the house. So even when playing at home, the Gretsch do everything I need. Still... when I take the Slingerland set out of the shelves and set them up to play, I'm filled with nostalgia. I remember my dreams of being on stage like Danny and Phil. I feel again the youthful thrill of wonder and excitement, and I remember how exciting it was to learn to play.