NextGEN gallery is one of the most popular plugins for WordPress. It has been used worldwide for creating multiple image galleries with captions and slideshow features. I use it for one of my client's sites as an "image museum" of sorts. See RikEmmett.com.
Now with this said:
IF YOU USE NEXTGEN GALLERY ON YOUR WORDPRESS WEBSITE, USE EXTREME CAUTION IF UPGRADING TO THE 2.X VERSION
WordPress has been upgrading its core to rely more heavily on jQuery - specifically a responsive admin bar that can be utilized on any device including phones and small tablets, and smoother slide and fade animation.
Photocrati has tried to keep pace with a massive upgrade that not only makes greater use of these animated features, but added more features to the management of the galleries themselves.
The response has been mixed to say the very least. The support forum on WordPress.org reports everything from extremely slow load times and random error messages to complete site crashes that required admin support from the hosting provider to restore the website from an archived snapshot. I tested the latest 2.x version on a WordPress 3.8 install (the latest release) and while it didn't crash the sandbox, I saw the slow page loads and error messages.
The leader of the development team issued an open letter to the WordPress community acknowledging the problems with an apology and an explanation of how to proceed next.
In SDOC's opinion, it's best not to make this upgrade. Consider the last stable release of this plugin to be version 1.9, which can be manually downloaded here.
So what did we learn from this WP plugin problem?
1) The "simplicity" of WordPress is not always so simple when you start adding on additional functions. Simplicity and ease of use on the front end for the user is always matched by increasing complexity and hands-on management for the administrators.
2) WordPress is not the ideal solution for every website, in spite of its popularity. I use it myself in some very complex installations (espcially RikEmmett.com and Atlantapanhellenic.org). But it also has big limits, especially in the frequency of major updates to its core and spotty quality control in plugin releases.
3) ALWAYS back up your files and database before adding or upgrading plugin functionality. Better yet, test them in a non-production sandbox site (if you have one available) before adding it to your "live" site. Most of all, ask your web host if they can automatically back up your site on their side on a regular basis (read: nightly, if you have a high-traffic site with frequent new content.) This advice has saved my bacon more times than I care to remember. If the worst thing you have to do is restore your database and reupload your files after a crash, consider yourself blessed.
Best of luck to your website owners and be careful out there.