My Take
It's a great way to make the information visually appealing and "hip". You can input text, video, images and more, arnd then arrange them as though you would a poster or a webpage. The graphics help make it visually appealing and offer a variety of themes. It does take a while to create your page though, especially if you're a perfectionist. However, the editing tools are very easy to use, so there's not a technical learning curve to get over. Also, the posters can take a while to load on slower connections. For example, I have a Glogster page embedded on my wiki and it can take a minute or so to load on slower connections. You can also provide a link to the page or embed it in an email or webpage. When my students use it for projects, they can easily share their creation with me or their peers by emailing the link.
They do provide a free Educator account, which is always valuable to teachers.
How I Use It
I've also had students create Glogsters instead of posters for presentations and was very successful. During a unit on the meaning behind vocabulary words in relation to Greek Mythology, students created presentations of a chosen word and were allowed to use Glogster as a visual aid. It was very easy for my students to create an account and figure out the editing options. It was very easy to take advantage of Glogster's options.
I also use Glogster to design pages on my classroom wiki. The Glogs help spruce up an otherwise boring wiki page. I can easily embed the poster into my page. I also create resource pages for the literature we read and I can include links to summaries, as well as embedded YouTube videos and pictures. Again, it can sometimes take a long time to load, but that is when the connection is slower than normal.