For me, it was the pinnacle of browser UI. Everything that came after that (including of course later iterations of Opera, which removed the MDI) has been considerably worse.
Perhaps a bit "bloated" for its time, but it was always pretty sleek to use. In fact I don't exactly remember why it never became mainstream. Maybe it had too many quirks or lagged behind others in terms of the new features, which... isn't entirely its own fault.
Looking at it now, it seems to have an identity crisis now. It looks like it has Whatsapp integration, among other things, which is the absolute last thing I'd ever want my browser to have an integration with.
The funny thing is that even though it included a lot of features I didn't use (for example, I never used the email client, not because it was bad, but I didn't use POP3/IMAP in general during that period), it was still much more efficient than all competitors in terms of speed, memory usage and even download size (at some point it boasted to be the only browser that fit in a floppy disk, although of course not for long).
I think the reason why it never became mainstream is that it was a paid product for a long time, while all other browsers were free. It eventually became free, but by then, competitors were too entrenched.
I also dislike Whatsapp but depending on country, it might be a necessary evil. In Spain for example, not having Whatsapp practically means forgoing any socialization with non-nerds. It's so ingrained in daily life that being without it is almost unthinkable for most people. Anyway, the current Opera doesn't have almost anything in common with the original one. Different engine, company, team, vision and even continent (owned by a Chinese company), only the brand and a thin sliver of identity remains.
(The sad thing is that I use it, because I still find it to be the least bad option for my needs...).
Also for a long time one of their main marketing points was that it had really small binary size so you could download it fast. They really spent a lot of engineering in making it performant
It's always worrying (and intriguing) when a small upstart decides to go closed-source when essentially the entire market is open-source.
Opera was sold to a Chinese company in the mid-10s and introduced suspicious stuff like a built-in free VPN. It is basically ignored nowadays.
The original CEO started Vivaldi, which is an excellent browser in terms of features - tiling tabs [0] were a favourite of mine. Unfortunately it is closed-source and chatty [1], but I still like on devices where privacy is not a concern.