Personally I really like the new iTunes 11. A clean grid of artwork with the gorgeous expanded view for albums. I think it is a very strong release. Good riddance to the sidebar.
The biggest gripe I had with the new version is about how search has changed. It used to be that you typed a search query and only what matched showed up - essentially filtering the content to only the relevant songs.
Say I type "clapton", the new way works like this:
A small pop over in the top right, under the search bar, with results from all different parts of the library. I don't see how this new behaviour is more efficient.
I was happy to find out that there is a solution to bring back the old behaviour. Click on the magnifying glass and uncheck the box to "Search Entire Library":