Hi Kevin,
It's been a while SOLR is being introduced and it's features are really impressive but I really do not have much success with SOLR because of following issues.
1) SOLR synchronization issues which comes up now and then.
2) SOLR licensing issue which was there in 4.0.x versions had impact its image very badly.
3) From my personal experience whenever I have added huge amount of data in alfresco and after doing customization at some phase I face issues related to SOLR and I really have no idea how to get though it(most of the times) So, I had to switch to LUCENE again.
4) I feel there is also lack of use-case demonstration of advance SOLR features which can be used in Alfresco.
This has been my story so far with SOLR. SOLR is powerful no doubt about it but it's just all of above points acting as major hurdle in its proliferation.