I am liking calibre quite much! I am finding that it is a very powerful database management tool just to keep track of data related to my books. I like that I can track more information like where I got the book from. I also like the search capabilities to more easily evaluate my library in various subsets. I like that it is so friendly with the Sony ereaders without having to touch the insides of your books. I can quickly update the covers in calibre and have it transfer over to the Sony thumbnail folders without having to do an EPUB to EPUB conversion or my own modifications to the files if I don't want to spend the time. Most of my conversions are EPUB to MOBI and vice versa to share files between my Sony ereaders and Kindle. I do occasionally use the Tweak EPUB feature, and I also found the Modify EPUB plug-in that can update metadata without the impact that a conversion causes. I have done a few conversions to add extra CSS for the Amasis font because of the italics/bold bug issue. It works ok generally if it's not a file that needs picky or complex formatting. I use Sigil if I'm worried about formatting. I'm probably not quite the purist that you are.
I think it will work ok for you if you continue to use the same methods to format your books as you do today and use calibre mostly as a data management tool. Hope that makes sense!