Have you ever passed on a recipe because you didn't have a pastry bag (or didn't want to clean the one you have)? What if you could just make your own with a plastic bag? Well, you can. And it really works!
Start with a large plastic zipper bag, carefully fill, snip off an end, and you're prepared to pipe. This tip works great for less-precise projects such as piping macarons or meringue, and, decorating cookies!
Do you make your own pastry bags?
→ Read more: How to Make and Use Your Own Pastry Bag at America's Test Kitchen
Elizabeth Apron fro...

1) I've been doing this for years.
2) you have to watch how much pressure you apply and how firm your mixture is or you'll burst the side seams of the plastic bag.
I use a plastic bag for decorating gingerbread for Christmas. You can make very fine, thin line with it. I learned this from my mom. She tried several types of pastry bags, but non of them was as good and easy-to-use as the simple plastic bag.
I agree with ladybug683 's second comment. That actually happened to me this weekend.
Also, use thicker plastic bags not ones from the Dollar Store. Cheap plastic bags are thinner and will also burst at the seams.
I just did this a few weeks ago to make little meringues. Super easy and it went a lot smoother than I was expecting.
My mom always makes hers out of folded triangles of parchment paper. She's a cake decorator and has always used those instead of traditional pastry bags for detail work. They're stiffer than bags and can handle more pressure! Now if only I could master the crazy origami technique she uses to make it work...
I've been doing this for years as well, but I'll do you one better: you can actually use coupler tips with this. Just cut your hole big enough to shove the actual tip through (but not the base) and then screw on your coupler on with the bag in-between.
And yes, no cheap sandwich bags! Grocery brand freezer bags are the cheapest yet heaviest plastic, I've never blown one out.
I've never used a real pastry bag, just Ziplocs (like everyone else said, no dollar store bags for this task), and, on one desperate occasion, an emptied powdered sugar bag.