Posters are an important tool to communicate your research findings to a larger audience. The format is different from oral presentations or full papers, and special rules (https://web.archive.org/web/20120612094621/http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.0030102?) for a good poster apply. Posters can be an important step before a full publication, although many posters will never (https://web.archive.org/web/20120612094621/http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=17296415?) be peer-reviewed and published.
The problem with posters is that they are second class citizens to oral presentations in most meetings. My personal experience with posters, both my own work and posters from others, has been mixed. In some meetings the poster presentation was a relaxed event (including beer and brezels in the last meeting) with good discussions in front of the poster, but often the poster presentation is not much more than a trick to increase conference attendance.
A recent paper (https://web.archive.org/web/20120612094621/http://www.aerzteblatt-international.de/int/article.asp?src=search&id=58801?) (link to english version) in the German journal Deutsches Ärzteblatt systematically interviewed poster authors and attendees at a German meeting. This meeting used the format of a moderated poster presentation. The attendance in the poster presentations was very low, but was valued by younger scientists and by the moderators. One third of the posters had already been presented at another meeting, an issues that touches the problem of duplicate papers that we recently discussed here (https://web.archive.org/web/20120612094621/http://network.nature.com/forums/harvardpublishingforum/954?) on Nature Network. In another study (https://web.archive.org/web/20120612094621/http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=17296415?), 12% of the posters had already been published as full paper at the time of the meeting.
In my opinion poster presentations are an important part of every scientific meeting. They should be taken seriously by using a competitive peer review process, including the rejection of abstracts that have already been presented or published. And they should be allowed enough space and time in the meeting schedule. Maybe we could also come up with new formats (e.g. this video in poster (https://web.archive.org/web/20120612094621/http://www.ncbi.nlm.nih.gov/pubmed/18246386?)) that make the poster presentation both fun and scientifically engaging.