"Semi-sweet" has a very broad definition in the chocolate world. These chips are at least 35% dark chocolate with a sugar content somewhere between milk chocolate (more sweet) and bittersweet chocolate (less sweet) depending on the particular brand. They will hold their shape when baked in a cookie or cake, but will also melt evenly when tempered or mixed with hot milk.
For general baking purposes, I usually use Nestlé Tollhouse chocolate chips. Their flavor carries a nostalgic quality for me, but I also truly like the particular balance of sweet and bitter that they bring to a batch of chocolate chip cookies and other baked goods. They tend to compliment a batch of chocolate chip cookies or muffins without stealing the show.
I've started buying Ghirardelli semi-sweet chips for the times when I want a little more from my chocolate. I find that these chips melt better than the Tollhouse variety and have a richer flavor. They're my choice whenever I'm doing something that requires tempering or for melting into batters and glazes.
What about you? What's your favorite brand of semi-sweet chips? How do you use them most often?