We've been spending some time trying to figure out what our team can get accomplished in the next 6 months or so. We have a feature list that we're considering, and we need to apply some numbers to it.
So, I decided to try using Planning Poker. I made up some index cards with values in dev-weeks, in the values 1/4, 1/2, 1, 2, and 4.
It has taken a fair amount of time to get through this process, but the numbers we got are a lot better than we would get with other methods, and more importantly, the dev team has a much better idea about what and how they'll be building whatever we are building (I apologize for being evasive, but it's a bit of an organizational disease right now).
Note that I'm not recommending all-up estimating. In general, I don't think you should be costing more than a month or so worth of work at a time, as the quality of the estimates won't be great. In this case, we needed a go/no-go number, and the estimates that we got were good enough.