We've been estimating a large new project for the last week or so using the "planning poker" technique. So far I think it's working out very well, although the biggest learning so far has been that you really need a solid task list before you enter this process. On backlog items where we already had task lists defined, we didn't have any trouble coming to consensus about how long it would take. This morning we entered into a backlog item without the task list already in place, and it led to an hour-long discussion that delved into design, etc. rather than getting any estimating done. While I understand that part of the planning poker process is group learning and making sure everyone is on the same page, we would have made a lot more progress with even a tentative task list rather than none at all.
Now we know.