We are now a good 6 months into guild achievements. As a guild leader, I think the concept, as executed, is great. Although we're casual and we run all content, trying to get certain achievements has provided us with incentives to level toons, level professions and to work together.
Every week, I post to the guild web site, a tally of what we're working on and how far along we are in finishing an achievement. Doing all the Burning Crusade heroic 5-mans made people run the regulars to get enough honor to get their keys. People went into instances they didn't know existed. Attendance at our retro raid nights spiked when we announced that we needed this run for the guild achievement. We're small so the 25-man achievements will probably elude us, but people take a look at what still needs to be done and they help make it happen.