Things I like about dyeing superwash yarn:
1. When you put a color somewhere, it generally stays there. Especially if the dye solution is hot.
2. When you need to do touch-up dyeing, it's much faster than when you have to touch up untreated yarn.
3. The colors are so vivid. Untreated yarn usually needs more concentrated dye solutions to achieve the same brilliance.
4. I like being able to rinse the yarn when it's hot, instead of needing to wait for it to cool down. Also, it rarely requires more than a single rinse. Pretty much all of the dye will bind to the yarn.
Things I don't like about dyeing superwash yarn:
1. When you put a color somewhere, it generally stays there. The dye strikes so quickly that you really have to work at it to make sure that all strands in a section are the same color. Pale or even (gasp!) undyed spots are not uncommon.
2. You don't get the same kind of color melding that you do with untreated yarn. Sometimes I'm really looking for a lot of shades that are in between two colors, and I don't get quite the same effect with superwash as I do with untreated.
3. My dye formulas that I develop first on superwash yarn need to be seriously tweaked before I can replicate them on untreated yarn. When I'm dyeing a mixed batch of superwash and untreated base yarns, this can make things a little screwy.