Gecko does its code cleaning according to the W3C standards rules as best as it can, not according to your page design and content intentions. So what you end up with after Gecko cleans house could be different than what you dragged into Composer. That early version of Composer was quite buggy and features poor. Since the Milestone 18 and PR1 releases, the Composer developers have been working on the HTML editor's Web page composing role. And their efforts have paid off.
Composer now is lots smoother and is reasonably well-packed feature wise.
However, there still are bugs and annoyances in Composer. And it still lacks some important features. There is lots more work to do on Composer. But, if the Composer development team keeps up the good work it has been doing, Composer should be in good shape by the time Mozilla 1.0 is released. And GoLive is not free.