I haven't owned a microwave for a long, long time and have come to think of it as useful only for melting butter and reheating coffee. But then we decided to have a microwave candy class, ripped from the headlines of this NYTimes article on the subject, and now I know how completely wrong I was about them. They make the entire process way quicker and way easier. I'm so totally sold.
If you're venturing into the wild world of microwave candy, you won't need a candy thermometer or a double boiler, but you'll want to perfect your cold water test technique (chart here) and have plenty of corn syrup on hand.
The cold water test is essential because it tells you when to stop cooking your candy. By dropping a bit of your hot syrup into cold water, you see what consistency your candy will be when it's cool. For caramels, you're looking for a temperature of 240°, aka firm ball stage--something that's malleable but not melty--while for brittle you want the crack stage, more like 300°.
When you begin a recipe, definitely make sure you know what consistency you're looking for at the start. Sugar heats up fast, especially in the microwave, so you're going to have to move fast.
As you can probably guess, as the sugar solution gets hotter, more and more water evaporates out of it, so caramels have a sugar concentration of around 87%, whereas almost all of the water has evaporated out of brittle, leaving a solution that's 99% sugar in the end. The concentration of sugar is what determines the texture of your final product.
Now that you've gotten the sugar hot, it's going to begin to cool immediately. With that, you're going to have to fight against the sugar's desire to recystallize into big chunks, which would make for less than awesomely textured candy. (There are, as with everything, exceptions to this, but for the most part you want to minimize crystal formation.)
How do you stop crystals from forming in your cooling candy?
There are a few ways you can do this, but one of the easiest ways it so use corn syrup, a magical and not-nearly-as-sweet-as-you'd-think liquid made from corn starch. Sometime around the 1860's, it was discovered that adding acid to corn starch broke the starch down into glucose molecules, among other things, and it because a useful and cheap sweetener.
In candy, the acid in the corn syrup breaks the sucrose down into its two constituent parts--fructose and glucose--thereby keeping it from forming big crystals. You can also fight crystallization with other acids, like lemon juice or cream of tartar, and with fats, like butter or cream. Each does a pretty decent job of keeping the sucrose from bonding to itself, and you'll see one or another of them in most candy recipes.
Of course, things will never be perfect. I've yet to make a decent looking batch of these pralines, but since the raw materials are pretty cheap and the microwave makes such quick work of everything, it's a fairly painless habit to have.
Here's the original piece to guide you: Harold McGee's microwave candy
And you can learn everything about sugar and sweetened condensed milk in On Food and Cooking.