Let me begin with a heartfelt thanks to everyone who creates, builds on and maintains free open-source software.  I rely on you heavily and generally buy upgrades or at least  kick in a few bucks to the developers of the ones I use for client projects.  I would also like to recognize that maintaining plugins for WordPress at the rate Automattic is kicking out updates has got to be challenging.  That being said, it is also hard being the person who has to explain to her clients why features on their site are suddenly not working.
Next-Gen Gallery is a heavily used and highly praised photo gallery and slideshow plugin for WordPress.  So much so that all of the sites that I have inherited have come with it in installed.  I am using it on the two sites I am currently building from scratch.  The benefits of using the same plugin so frequently are many.  It is one I know the ins and outs of and the best ways to make it bend to my will. The risks are what you would expect when one comes to rely too heavily on one single product.  When it fails, it fails everywhere.
The biggest issue is with one of my sites that is currently live.  A feature that has literally worked for years has suddenly stopped working,  And unsurprisingly, people have noticed. I did everything I could think of to resolve the issue.  I checked the support forums and learned that the problems I am having are happening to many others who updated their sites to WordPress 3.6.  I double-checked with the guy who developed the theme to see if there were any work-arounds I was missing.  I have thoughts about moving back to a previous version of WordPress, but then I would also need to go back to previous versions of several of the other plugins on the site.  That would come with its own set of risks that, at  least to date, do not justify the time or effort.
The second biggest issue is with a site that is quite frankly, overdue.  There are good reasons for it being behind schedule.  The client shifted some priorities and I hit a few technical challenges, but it was almost ready to go live.  That is, until Next-Gen kind of took me back quite a few steps.
So now I find myself obsessively checking the support forums and awaiting Next-Gen’s updates with bated breath.  In the interim, I am doing everything I can to move my other projects forward.