I started playing bass around 3 years ago and one of the first rounds I tried were Rotosound 66, among other common brands (D'Addario, Ernie Ball, Dunlop etc.). I didn't have any experience or reference to understand how good or not they were, but I remember I liked them.
During the last years I changed many type of strings in order to understand the differences and find what worked best for my style of playing. I've found home with nickel rounds and stainless steel flats and that's what I used for the last year. I tried many different brands of course but my "formula" has been the same.
Due to recent changes of needs, I wanted to try stainless steel rounds again and Rotosound was the first brand that came to my mind. At the end of the day, it's a classic.
I've put a set of 45-105 RS66 on my Fender Jazz and immediately noticed how much different they are compared to my other strings.
The good: tone is focused, tight, extremely bright and snappy. Good balance between strings volume, tension is on the lower side (imho, please discuss?) and definitely not super stiff, frequency spectrum seems uniform. Playing with harmonics is on another level with these strings and stainless steel in general. With proper technique and EQing you simply get EVERYTHING with these strings.
I must say that coming from Sunbeams I needed to get used to the Roto's EQ, because it's different and can be deceptive. Sunbeams are a little bit mid scooped with plenty of lows and a good amount of highs, while the RS66 sound more "flat", with less accentuated lows and more highs, which can be easily harsh depending on the bass, EQing etc.
During the first couple of days I couldn't hear my J bass very well when practicing and playing over songs, but now I assimilated that sound and it works pretty well in the mix.
The bad: bad/dull E string right out of the package. As far as I know it happens pretty often. I wrote to Rotosound and they'll send me a new set for free, which is something I really appreciate. Now that the strings are mellowing out the difference seem to be less prominent, but the E is still dull.
They're really abrasive under the fingers. I didn't test other stainless steel rounds recently so I can't make a comparison, but they're rough to the point I developed some callosity on both my hands in a couple of weeks. It didn't happen in the last 25+ years playing guitar, except when I was playing acoustic only. It didn't happen even when I switched to bass, to be honest.
Honestly, I don't mind the pain and callosity. Eventually they will disappear and my hands will adapt perfectly, I'm pretty sure about that. But i must say they're rough.
I'll avoid comments like "too much treble!" and "too zingy!" because that's what you get with stainless steel. I'm acquiring a new taste, let's see how it goes.