In anticipation of our trip up to the top of Mt. Lemmon (7300 feet climb, 61 mile round-trip) I got a new 12-27 cassette, and a 38t chainring on my standard crank. The drop from 39t to 38t may seem small, but this was a long climb for me and I wanted every benefit I could get. I've got a 50/34 compact crank on order now, and I think it will really set me up.
Also replaced the chain at the same time as the new cassette. During that ride on Saturday I just kept thinking the new gears were money well spent.