The technology really was great. Their understanding of object orientation was superior. The developer tools were wonderful. The user experience was generally a delight. We could develop custom software in a fraction of the time of people using the tools of the day. NeXT had a true vision of the future.
However, what they didn't have was much understanding of economics. The only reason that NeXT wasn't a complete commercial failure was that Apple's board wanted Steve Jobs back. If not, Apple might instead have bought out Be. And if Apple had succeeded in developing their own next-gen OS, both NeXT and Be might be minor footnotes these days. Even prior to the deus ex machina buyout, NeXT was on a slow and steady path to failure. They'd gone from an integrated hardware vendor to an OS-on-other-hardware vendor to a dev-tools-on-other-OSes vendor, and it's not clear that would have worked either. Once the acquisition was announced, they promised to take care of the people who had stuck with them and then did jack.
I took a few lessons away from my time with NeXT. 1) Just because I thought something was technically superior didn't mean it was commercially viable. 2) Being too far ahead of the market is worse than being behind it. 3) Never trust a "visionary leader" to look out for you, no matter what he says. He's in it for himself and the vision; the little people are expendable.
But you're definitely right that it made using other stuff painful. I stopped doing GUI development altogether rather than shift to Windows, which was incomparably awful by comparison.