When I first started knitting, I made swatches. For about a year, I just made swatches of different fabrics, because I was knitting for the therapeutic value. I didn't want to make anything in particular. (I wish I had kept all those imperfect swatches, because they would be a great visual display for beginning knitters, who always seem to think the scarves they make at a Sip and Stitch party should look perfect.)
When I finally started making hats, I didn't pay attention to gauge at all. Thank goodness I was working from a really good pattern and that the things were felted, because they did end up fitting. Later on, it caught up with me. I spent my whole Christmas holiday knitting a hat from beautiful blue Scottish wool, and it was huge. I had to felt it so that I could even wear it.
While working on Lesson 2 of Basics this week, I discovered, post-blocking, that I had made a mistake on Swatch 4. One of the decreases came a stitch early. So I had to re-knit that one. I had already decided I didn't like the way the swatches knit on US 8 needles looked, so I went down to US 7. The difference in size on the two swatches after blocking is significant, as you can see in the photo. The difference in width at the bottom is almost an inch. This is why I now always make gauge swatches!
I love this super detailed video by Cheryl Brunette about the gauge swatch. This video is a math demonstration I found useful when I first started paying attention to gauge.