I spent a year building weekly and bi-weekly emails across 7 brands for a Fortune 200 company with each one going to between 2 and 7 million subscribers. Here’s what I can tell you:
1)Yes, you have to use tables. It’s the only way to get a consistant display across several different mail clients.
2) You can you some embedded styles in the header, but inline will be most of what you get to use.
3) Get used to nesting. The more complex the layout, the more you’ll have to do it.
4) Microsoft will be the bane of your existence. This will look great in every client except specific versions of outlook, and there’s often little rhyme or reason to it.
5) This is not a plug, but Litmus is a great tool to preview your content in almost every mail client at once. It can save you a lot of time and is great for testing. I’ve used several others, I like Litmus the best.