Recommendations: Adonit's $90 Writer (iPad, iPad 2) sports a keyboard that's a bit cramped, but the keys are good, the clever design lets you choose a wide range of angles, and you can quickly pop out your iPad and place it in portrait orientation or use it on its own. The Writer's cover also supports the iPad 2's magnetic sleep/wake feature. Zagg's $100 Zaggfolio (iPad 2) supports only a single angle, and its keyboard is slightly cramped, but the actual keys are some of the best around, and they're arranged in a standard layout. The Zaggfolio also provides solid, all-over protection while still being one of the thinnest folio keyboards on the market, and it's available in multiple colors and materials.
Honorable mentions: Apart from the Writer and Zaggfolio, most models in this category are simply bulky leather (or faux-leather) folios with a disappointing keyboard tacked onto the inside of the screen cover. Of the 16 other folio keyboards I tested, two stand out. The best is Belkin's Keyboard Folio for iPad 2 (iPad 2; $100), which features a good-quality keyboard that you can flip behind the iPad when you want to use the tablet as a tablet; the Keyboard Folio for iPad 2 also gives you a decent range of screen angles (though no portrait mode). Kensington's KeyFolio Pro Performance Keyboard Case (iPad 2; $100) has a pretty good keyboard and lets you rotate your iPad into portrait orientation. However, the rotation mechanism adds bulk to the case, and the single prop-up angle is too vertical for my tastes.
Clamshell (laptop-case) keyboards
These models hold your iPad inside a hard-plastic, clamshell case that unfolds like a laptop. The downsides to clamshell keyboard cases are that they tend to be quite bulky, and that all of the models I've seen use smaller-than-normal keys. But the quality of those keys is often a bit of a step up from that of the average folio-case keyboard; clamshell keyboard cases offer the most protection for your iPad; and their laptop-style design works well for typing on your lap. Most clamshells hold the iPad in landscape orientation by default, while making it relatively easy to remove the iPad and prop it in portrait orientation. However, many make it difficult to use your iPad as a tablet when you're not typing, my recommended model being a notable exception.