Having recently started development with Android, I noticed that the emulator takes ages to load up initially.  My original thought was that testing a change would take a very long time.  However, this is not the case.  If you launch an emulator from the Eclipse SDK, you must remember not to close the emulator window.  When you make changes and relaunch the application your changes will automatically be installed on the existing emulator.  This is something that wasn't completely obvious to me when I started.

If you want to test your app on a different device format (new AVD) you'll have to wait for a few minutes while the new emulator loads.