NSIS is the installation SDK used by, among others,
WinAmp. The output packages it creates are much smaller (less overhead) than comparable
InstallShield packages, but the downside is that there is much more for the install programmer (me) to do to create one. The InstallShield IDE lets you automatically insert event handlers, functions, dialogs, and files to install; all of this (apparently) has to be manually scripted in NSIS. The tradeoff in size may or may not be worth it. I have a requirement to make the next version of the installation package as small as possible, and if I can lose the 1MB of overhead of an InstallShield package I'll be well on the way to fulfilling that requirement.