It took a few evenings of concerted attention to get it to work the way I wanted with a few domains. And even after working through the initial round of problems there are other problems to work out.
One example is when you have multiple top level domains (in my case davidherron.com and socialmedia4good.com and peaceguide.com) you're only logged-in to the primary domain. At the moment I'm logged-in to davidherron.com but switching to socialmedia4good.com I'm logged-out. Obviously the culprit is the domain= attribute on the session cookie. But I don't see a way to set that cookie. Logging-in on the secondary domain does not result in being logged-in.
Another is the notifications I get via the Messaging module. Originally the notifications arrived with correct URL's (http://davidherron.com/...) and having no base_url value in settings.php. But a couple months ago the URL's started saying "default" rather than my domain name, and the fix was to set the base_url in settings.php. But with Domain Access the base_url has to be left unset, again. And again the notifications have URL's saying "default" rather than my domain name.
The core problem is, as I understand it, that Domain Access must "integrate" with modules so those modules become enabled for multiple domains. As noted in a previous post, the Context module doesn't appear to be aware of Domain Access, whereas it would be really cool to have a Context trigger based on the current domain (please be patient about incorrect terminology).
Maybe this is a failed experiment that I need to undo. What I wanted out of this was partly to explore the module (I do have a site where it could be very useful), and partly to decrease the number of Drupal instances I'm managing. By combining multiple "sites" into one Drupal instance it might decrease the time I spend on administration. But it comes with a cost of greater complexity that might not be worthwhile.