I recently bought a used Kobo Clara HD and installed koreader on it. So far everything works perfectly. I haven't really tested to see if using koreader drains the battery much faster, but I really only read at home so I'm always near a charger.
I use Calibre to manage all of my ebook files and with koreader have it set up so I can transfer books to my ereader wirelessly.
The newer version of the Clara HD is the Clara 2E, but unless it has improved there have been a lot of users reporting freezes and crashes when trying to use koreader.
Also koreader is not the same as installing a custom ROM on a phone. When you install the custom ROM every time you reboot your phone it automatically launches the custom ROM you installed. With koreader the stock software is still there and if you reboot your ereader you have to click on the koreader book again to launch koreader. But I rarely reboot my device.
The mobileread forums are a good resource for koreader and other ebooks stuff. That is where the devloper of calibre, kovidgoyal, posts and where the developers for koreader post. You can find installation instructions as well as all the files you need over there.