Mantine is brilliant, I can build anything in it quickly and then extend it or completely customize the theme or individual components, but there is a learning curve. I would not call it a giant learning curve.
I love Mantine. It’s easy to use, full featured with lots of components and helpers, and yet if you need to, you can use it headless and customise it too (I’ve never bothered though; I did add my own additional spacing/sizing options via the theme support though)
Any direct comparisons to base UI? I've been pretty satisfied with base UI so far, but my usage has been very basic so I'm open to switching if Mantine offers something better.
They have very similar concepts and goals, only Mantine is much more full-featured (without being messy and while maintaining flexibility). I'd suggest if you hand-craft most of your site, but need a well-thought-out complex widget, use Base UI -- but bear in mind Mantine also offers unstyled components.