Wow, these were a huge surprise! I expected a nickel version of the steel 66 Swing Bass but these are COMPLETELY different strings. They feel totally different and they sound totally different than the steel 66s.
These are probably the warmest and fattest sounding roundwound strings I've ever used. I could hear it even before I plugged in. These strings sound remarkably full even unplugged! This fatness comes at a cost though. These are definitely not the clearest sounding strings I've used. In fact they are just barely on the right side of muddy. But I love them! They start out surprisingly bright for nickel strings (sounds more like nickel plated to me) but mellow out after a while. They never sound harsh though, even when new. The highs are very smooth. The mids are beautiful with focus on the lower midrange, hence the fatness and the bordeline muddy sound. The lows extend deep and are pillowy rather than tight.
The texture of the strings is totally different than the steel 66s. These strings are a LOT smoother and considerably less stiff. Very comfortable.
They also seem to last longer than the steel 66s. Or rather, they mellow out more gracefully.
Highly recommended if you're looking for warm, fat sounding roundwounds. Avoid if your priority is bright, clear and tight.