I do all of my XHTML/CSS in Dreamweaver although for quick mods I usually use Notepad++ because it's not nearly as heavy.

I use Dreamweaver for the following reasons:

• Decent Design Preview
• Code completion
• Great CSS integration
• Tabs (Stupid I know)
• Easy interface

Plus now that it's an Adobe product, I expect they'll start integrations with Photoshop (ie - in Flash you can now import a layered PSD).

Best Regards,
Patrick