Are you talking about the Rotosound Steel 66 (orange packaging) or the Rotosound Nickel 66 (blue packaging)? I love both but they're extremely different so you need to specify which one if you want accurate answers.
Taste in strings can be highly subjective of course but here MY take on the Roto 66s:
Steel 66
The Steel 66s are very bright but it's not really an airy, zingy kind of bright. It's more about ultra-aggressive upper mids. More ROCK than slap. The brighness won't last long though. Personally I like their broken in sound (very 70s british rock) but most people buy these for their ultra-aggressive brightness (think Entwistle or Chris Squire) and they won't stay that way for long, 2-3 weeks at most. Some would say they need replacing after every gig. This is the main criticism about these strings, plus the fact that they can be a bit rough on your fingers. They're probably the roughest feeling strings for sale today.
I often see people compare the Roto Steel 66s to DR Hi-Beams but I honestly don't understand why. They are both bright strings but they don't sound even remotely similar. The Hi-Beams are kinda scooped, the Roto Steel 66s are extremely mid-forward. Or to put it another way: Personally I think the Hi-Beams sound more like 90s death metal while the Roto Steel 66s sound more like 70s British prog. They also feel completely different and have different construction (round vs hex core). Personally I love the Rotos but I'm not a big fan of the Hi-Beams (the DRs will stay bright for a LOT longer though).
However, even though I love these Rotos I also find them to be a "speciality" string for when I need THAT sound. I usually have them installed on a couple of basses (usually a T-bird + my Rick 4003 + a P-bass) but unless I only worked within one specific genre that needed this specific sound I wouldn't use them on my main instrument. They're excellent for what they do but not very good allrounders IMO.
Nickel 66
The Nickel 66s are totally different. They're much better allrounders and I like them a lot. They are a bit bright (metallic clank rather than zing) when new but mellow out nicely for a very warm and well balanced sound. Brightness wise I would say they're slightly on the warm side of medium. I have seen some people (and dealers) on this forum say the cheaper Rotobass strings are the same as the Roto 66 Nickels. That's simply not true (and I got this confirmed directly from Rotosound). The Rotobass strings sound closer to cheap nickel plated strings like Fenders or D'Addario Xls and don't last very long before they go COMPLETELY dead. The Roto 66 Nickels are much better quality, last longer, sound much nicer and have a lot more character too IMO. I could easily use the Nickel 66s on my main instrument and be happy. They're probably my number 3 favourite roundwound after Pyramid Pure Nickels and Thomastik-Infeldt Superalloys (but I love all of them).