I used canned pumpkin pie mix (About 56oz of Libby's) in my first pumpkin ale in 2010, and it turned out great. I did the Austin Homebrew extract kit at the time, and additionally added their spice packet. It was like liquid pumpkin pie. I remember that the AHS kit specifically states NOT to use pumpkin pie mix because they thought the spices in that mix along with the kit's spices was overkill, but I thought it tasted great.
In 2011, I made an all-grain pumpkin ale with organic pumpkin puree and pumpkin pie spice, and the pumpkin flavor was much more subtle, even though I added the recipe amount of spice and then some.
Late last winter, I then went back to the AHS recipe, though I made it myself at my LHBS and converted it to AG.
In all cases, I did the pumpkin meat in the mash and the spices at flameout.
Just note that either way, you really get what you think of as the pumpkin FLAVOR from the spices. The pumpkin meat itself adds much more of a thicker protein mouthfeel, but pretty much nothing else. For this reason alone, I wouldn't do it in secondary, I'd stick to the mash.
I've also read that peeps have had much better luck actually getting good pumpkin flavor from the meat of yams and sweet potatoes as opposed to pumpkins, so I wouldn't hesitate to actually go that route.
Let me know if you want any more of my experiences. I make a pumpkin every year right around mid June and pop the first ones when football starts!