Basically, any launcher in the market (ADW, LauncherPro, Go EX, etc.) will replace your homescreens. They have some advantages over the stock launcher, such as changing the number of rows/columns on your homescreen, resizable widgets, themes, different screen transitions, as well as some others (those are basically the standards). Unfortunately, you will lose access (at least while using the alternate launcher) to most of the stuff HTC integrated into their Sense UI. Feel free to download and try them out, ADW and LauncherPro have free versions that while not all features are available will give you a pretty good feel for them and Go EX is free and full featured, but has lots of add ons, some of which you may have to pay for. If you don't set a default, you will have to choose which launcher to use everytime you hit the home button, and you can always clear the default by going to menu>settings>applications>manage applications, scroll down to which one you set as default, press it, then press clear defaults. This, in itsself, is, IMHO, one of Android's greatest strengths, you can completely change the look and feel without having to root your phone.
there is also an app called home switcher that will create an icon to easily switch between your htc sense launcher and a different launcher (i have used ADW and really liked it). i love the app drawer scrolling options and home screen scrolling options in ADW. however, i always end up going back to sense because i love the widgets built in to htc sense and you can't access those. i found myself using "beautiful widgets," etc. just to try to make it look like sense again....lol.
I was having a lot of lag and redraw issues with my Samsung Droid Charge (even after the GB update). I switched over to GoLauncher EX and haven't looked back. For whatever reason, it got rid of the lag issues and gave me so much more freedom to set the phone up like I want to. Once I decided I liked GoLauncher, I sprang $1.99 for the Black Gloss Theme which I like so much better than the stock TouchWiz theme that came with the phone. GoLauncher has a lot of free add-ons and widgets to try out too.
As mentioned above, after you install a new launcher, your original stock launcher is still there. Once I decided to keep GoLauncher, I went back to TouchWiz and deleted all my widgets on that launcher to prevent them from running in the background. I don't know if it really saved any battery life, but with the Charge you learn to never pass up an opportunity to save juice!