Hate them. I have some of the muffin cups, and some silicone liners (Silpat and others)
Instructions say not to put the muffin pans on a cookie sheet when baking--but those things are too flimsy to pick up. Especially when hot. And try positioning the muffin pans on your wire oven racks so they're all level without a sheet supporting them.
Instructions say not to grease or spray the pans, nor to use anything sharp like a knife. So I lost fully half of each muffin when I tried to removed them--they stuck so badly.
There are some questions about the safety of using silicone for baking.
I'm very sensitive to smells and tastes. I cannot eat food baked in or on silicone because of the 'plasticy' taste it imparts to the food (not everyone is sensitive to this, but many are).
I like a nice crisp golden crust on my baked goods--you can't get that in silicone.
Hard to store the silpat things--you aren't supposed to roll them, because it can damage the interior fibers.
You'll find, though, that there are as many people who like silcone bakeware as who hate it. I'd suggest, if you asked because you're thinking of buying some, that you find a friend who has some and borrow it to try out before investing a lot of $$$. Who knows, if they feel as I do, they may just give it to you. If you can't do that, I'd start with just one piece to try, before you buy a bunch of something you may end up shoving in the back of the cupboard and never using again (of course, with it being flexible, you can squash it, and it won't take up a lot of space, if you never use it again--LOL) Oh, and I'd strongly recommend against giving it as a gift unless you know for certain that the person you'd be gifting it to has used it, likes it, and doesn't mind that there are health questions that have yet to be answered with the stuff.
Now, I will conclude with this--I did find one good use for those muffin pans. I put paper liners in them (that eliminates the sticking problem, and the chemical taste leaching into the food), and I use them in my microwave to make quick, individual cheesecakes--only takes 4-5 mintues to bake a cheesecake muffin in the micrwave.