Yes, I'm very happy with it. I've been "stuck" with it for months. (I can't re-flash the original firmware). If you're happy with FBReader, you should be happy with OpenInkpot. (E.g. no DRM. You can install FBReader on your computer to give it a try)
The UsersGuide on the wiki is accurate, at least about the book-reading features you can expect. (The images are a bit unreadable, but they get bigger if you click on them).
I wouldn't use Calibre with it, at least to start with. Calibre is somewhat unsubtle when it comes to converting e-books. The default settings appear to force justification but prevent hyphenation, which can get rather ugly.
There's one crash bug I noticed in 0.2 - FBReader crashes when a hyperlink is too wide to fit on the screen. It's fixed in the unstable version 0.3-rc1.
The hardest part is installing OpenInkpot. As the FAQ says, it's not possible to "brick" the device. But some people find they can't re-install the original firmware. Nate had to try several times before the installation succeeded; we don't know why. Judging by the forums, many people seem to be using it. AFAIK no-one has reported complete failure. So any risks are currently theoretical.
I'd worry more about the possibility of being stuck with the original firmware :-). The original firmware may have DRM support and a sound recorder, but apart from that it's strictly inferior.