I'm curious about all these launchers, but have no idea how you might choose one. What do they do different from the stock launcher? What do you like about them? Do they change the number of home screens, or how stuff sits on the screens?
As someone else said you can see descriptions and screens on the market, but here's a quick run-down. Alternate launchers allow you to change the look of the ever-present icons at the bottom of the screen (aka the dock) by choosing different icons, having them launch different programs, and changing the number of them. The stock Moto launcher with its white background gives you four, and only three are changeable (the app drawer is not). Compare that to Go Launcher or ADW, which both allow for five icons and the look is completely up to you depending on settings and theme.
In addition to the dock changes, these launchers allow you to add or remove home screens, move them around, and change their behavior (transition animations, whether the screens wrap or stop when you hit the last one). You can also alter the behavior of your app drawer in a similar manner, in addition to customizing icons. Some come with all-new widgets to play with, and most have active developer and community support with bug fixes, new themes, and new widgets.
If you decide to play with them, I suggest getting a launcher swapping app like Home Switcher or HomeSmack so you can easily compare them. Just remember that if you have more than one installed, they will be running in the background even if you're not currently using them (so I don't recommend keeping more than one beyond stock once you find what you like).
do these launcher's cause a battery drain?
They don't noticeably impact battery life for most people. A few Market reviewers might complain, but for the most part you won't see a difference. See my point about running multiple launchers, however; I finally stripped my original Blur set-up of widgets because Moto's widgets were running in the background sucking battery and resources even though I wasn't using any of them under Go Launcher.