I do it every year... my carved pumpkins stay on display only 1-2 days at most. After that I roast them in chunks in the oven, peel off skin and grind them in my food processor, store in ziplocs in the freezer and cook soups/pies with it. They taste like every other squash.

In my experience it's not worth the trouble. Those decorative pumpkins weren't grown with consumption in mind, so they're very bland and starchy. It's nice that you want to re-purpose them; have you considered painting them for your holiday table scape or gutting & carving them to use as candle holders or flower vases? I bet Martha Stewart is full of ideas.

If they're small, there is a chance that they are sugar pumpkins. It wouldn't hurt to cut them open and take a look inside. If they're not too stringy, go ahead and cook them as recommended by Bilbo Douchebaggins.

Do a Google Image Search for "sugar pumpkin" to get an idea of what they might look like compared to a carving pumpkin.

I came up against the same dilemma the other week and I went ahead and cooked them. The larger pumpkins had a more floral fragrance than sugar pumpkins as they cooked, but the puree was moist and had a great color and texture. I squeezed out the excess water and turned them into some olive oil pumpkin bread using the crisper whisperer recipe, but I switched the cloves for cardamon, and it was stellar: http://www.seriouseats.com/recipes/2009/10/the-crisper-whisperer-olive-oil-pumpkin-bread.html?source=thekitchn
And I even had a little over a cup left to make some pumpkin and greens mac and cheese for a holiday party! Which was also a hit!

Do it!!! Cook them!!!

I tried to cook 2 of mine after about 10 days and I ended up throwing it all away. Like HeyYo said, the texture was strange & they had no flavor. Guess you have to cook it to see what happens.

At what temperature did they sit? If they've been baking in the warmth that means bacteria could have seeped in. If it's cold and within the expiration date of a pumpkin (and it's not a refrigerated only item), it's could be okay. If you're looking to be less wasteful a compost pile or bin is great. Stuff like this can always be made into soil for next year's pumpkins. :D

A tip: DO NOT steam them -- roast them, instead. They'll pick up even more of the abundant moisture they already have and you'll have a soupy mess.

Another year, buy culinary pumpkins for your decor. Then you'll be sure they serve two purposes well!

Not sure what your definition of 'decorative' is but I always cook with mine when I change my outdoor display from fall to winter. Never had a problem (30+ yrs) and we often have some rather warm days throughout Nov. Roasting is pretty much hassle free so I'd roast 'em & see. Pop a casserole in the oven while you're roasting them & the worst thing that can happen is nothing.

Depends on what kind of pumpkin it is. Mine were a bit darker and rougher skinned than usual. The owner of the pumpkin stand told me to knock off the stems and roast them. No cutting necessary at this stage. I ***think*** I roasted them at 350* for 50 minutes. The skin peeled off nicely after that. The pies had a slightly stronger and better flavor than - pardon me - canned pumpkin. Also made good soup with them that tasted about the same as when I used a butternut squash.

Any pumpkin can be cooked, the bigger ones are just not as flavorful. However, they make terrific edible vessels for all kinds of stuff and when you eat whatever is inside them, you can get chunks of pumpkin in it.

For instance, I always do my Thanksgiving stuffing in a pumpkin (it gives me the option to make a vegan stuffing if I have guests with that inclination.) There is also a famous Argentine stew, Carbonada Criolla, that is served in a baked large pumpkin (don't try to cook it in the pumpkin no matter what the legends say - it will fall apart...and serve it inside a container that can hold the stew if it does fall apart.)

In both situations, the filling is flavorful enough that the pumpkin takes it on, and the slight flavor of pumpkin enhances the filling.

You CAN cook with them, but they just don't provide as much flesh or flavor as a normal pie pumpkin would.

I have used mine to cook baked mac 'n cheese in. It adds a little pumpkin flavor to things, and makes for a playful, fun presentation. I pre-roasted the whole pumpkin a little bit first, since it needed longer than the mac 'n cheese to cook.