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".
Now we're back to relevant search results within the active view tab, displaying all the content that is relevant to the search query. What a relief.