When I learned to sew I didn't get the best explanation on what the stuff was, so I learned as I went. I trial and errored a lot of different things, learned about my favorites, what would work in a pinch, the virtues of each, and so on. I have a few favorite stand-bys I buy by the bolt, and some I buy as necessary. I can't stand cutting it and fusing it/sewing it, but I wouldn't dream of making a hard-wearing bag without the stuff. There are tons of varieties with tons of purposes, but I'm going to focus on those I find best for bag making.
I'll be honest--sometimes I skip it, depending on the bag. All of the easy totes I wrote tuts for over the summer? None have interfacing. If I'm using home dec weight fabric I'm more inclined to skip it there, too (but not always). However, if I want a more structured bag that is going to get a lot of heavy use I stick some in for sure. I'm one of those folks that carries one bag per season--I don't care for changing my bag to match this or that (OK, I'm too lazy to do it) so I need my stuff to see me through. Using the interfacings I'm going to tell you about I've never had an issue with excessive wear and tear.
Remember this bag? It's made out of Walmart fabric and interfaced well. I've used it as my fall bag for 3 years in a row (I love it that much)--and the only spot there is the tiniest bit of wear that you have to reeeeaaallly look for is on one of the outside corners.
The products I use are all made by Pellon. It started as a coincidence as that's all I could find when I started sewing, but ended up being just fine as I love love LOVE their products. I've tried going the cheap, generic route and regretted it. When it comes to interfacings I suggest you buy the best you can afford (if that's an option). But if you must go cheaper then do so--it's better than omitting it altogether.
Guys? I'm getting tired of typing 'interfacing,' so it's going to be IF from here on out...
First up, I always have Shape Flex on hand. It's a woven, fusible interfacing that comes in white and black, so you have some color options depending on your fabric colors. It works beautifully with quilt-weight fabrics--it maintains the drape of the fabric, strengthens it, doesn't bulk things up, and is so easy to apply! You can press right on top of it and it won't melt or burn (unless you put the iron down on it and walk away for an hour). My biggest complaint with it is that it only comes twenty inches wide--I'd love it if they made it the width of standard fabric. Maybe one day... I purchase this by the bolt from Joann's. You can definitely find it elsewhere, but I like to tack on one of those 50% coupons they send to make it a real steal. I bought a twenty yard bolt back in March and just now am getting close to replacing it. They also sell sew-in woven, if that's more your inclination.
My second love is Thermolam Plus Fusible Fleece. It's a needled-fleece with adhesive on one side. It comes in white, but I'd love it if they released it in black, too. This is my other standard IF by my sewing table. I use it in almost every single bag I make. It keeps the bag soft and squishy, yet strong and structured. I also buy this from Joann's with one of their coupons (because I don't like paying more than I have to for this stuff--it's such a necessity but I hate paying too much for it. And here's me telling you not to be cheap, right? LOL). For this one you must use a press cloth if you iron right on top of it or the stuff will melt and stiffen and you'll say bad words and such. I tend to put my fabric side facing up and fuse it that way. They also sell this as a sew-in which is just as nice as the fusible.
Almost every bag you see on my blog has those 2 interfacings in it (unless noted otherwise). I stand behind these products completely (and I'd stand in front, under, to the side, wherever if I had to :)
Third, I keep a much smaller bit of Peltex on hand. This is quite a firm interfacing for when you want to give bags a boxy structure that will keep their shape on their own. I use this much less often than the other two as I like pluffy squishy bags more, but is my go-to when I need something boxy and stiff. I normally buy the sew-in variety, but they do have this in fusible and even double-sided fusible. This can be quite bulky in the seams, though. What I like to do is cut the Peltex smaller than the size needed by the width of the seam allowance on each side, and then iron the full-sized woven on top of it. It's held in place on the edges, where you want it to be, and when you sew it together there's no extra bulk in the seams. BUT your project will still be nice and structured. This helps when you have to do that top-stitching around the top edge of the bag where all the seams and layers come together--saves a few needles, that's for sure!
This bag can stand up by itself thanks to Peltex :)
If I've got no Shape Flex and the store is out and I neeeeeeed it I will buy the medium craft-weight stuff. It looks a little bit like fibery-paper. That too comes in sew-in and fusible, but I tend towards the fusible. I used this stuff when I started making bags and it does hold up quite well--I just fell seriously hard for Shape Flex when I found it. Sometimes the light-weight stuff works well, too--I tend to stick with that more on small purses that won't bear a lot of weight, or something that might not be used often.
As you can see I tend towards using fusibles--I dislike pinning all those layers together or sewing around the edges and all that stuff unless I absolutely have to. I will always try to figure out a way to make something work with a fusible before I do the extra work. Some people don't like them or have no luck with them, and tend towards sew-ins. I've used both and haven't experienced any major differences that would make me lean towards one or the other (besides my aforementioned laziness, of course). The only instance I can think of in which fusibles absolutely will not work is if the fabric you're using has a fairly open weave--you will have a sticky mess on everything from the adhesive working its way through (either on your ironing surface or on your iron--neither are fun to get off). If you've got any wisdom in the fuse/non-fuse area, please enlighten me in the comments. As I'm largely self-taught I have more learn-as-you-go-knowledge on this particular subject.
If you make bags or are interested in getting started, give some of these a whirl. I don't think you'll be disappointed at all. If you've got some goodies you use that I haven't mentioned please let us know in the comments--I'm always looking for new tips and tricks to improve my projects.