I've been asked that question twice. Both times I said yes. The first time was a few years ago due to an electrical fire in a Seneca I was instructing in. I was near the airport and landed uneventfully. The second time was today.
We were descending into Portland, deviating around some rain showers, when the flight attendants called to say there was smoke in the cabin. Shortly thereafter, the lavatory smoke detector went off, and then smoke became noticeable in the flight deck. I got my oxygen mask on, the emergency was declared, & ATC gave us direct to the airport. I was the pilot flying, so I mainly concentrated on getting down quickly while the captain ran the checklists, talked to the flight attendants, coordinated with ATC, etc.
Before landing, we ventilated the cabin in accordance with the checklist by turning off the bleeds and opening the forward outflow valve. The smoke seemed to be dissipating a little bit, & the plane certainly wasn't on fire, so we planned on a normal landing & deplaning. I made a good crosswind landing on 10R & we were followed to the gate by the crash trucks.
I left before all the evidence was in but it seems like a seal blew in the engine & allowed oil vapor to enter the packs, causing the environmental system to spread the smoke. Thus, it wasn't a major deal, but anytime you see smoke in an airplane, you assume the worst and get on the ground now. We were pretty successful at doing that. It just took a while for my heart rate to come down.