SmugMug to Flickr. I switched to SmugMug from ClubPhoto years ago. ClubPhoto became kludgier as time went by, and I reached the limit of my patience with them. ClubPhoto went under not long after I jumped ship.
I've been happy with my SmugMug. It allowed me to upload (and others to download) full resolution images. Navigation was always easy. SmugMug was a subscription service; there were no annoying ads for visitors. SmugMug has always been Mac friendly, and added tools that allowed for easy Facebook and Twitter sharing.
So why the migration to Flickr?
Apple has "blessed" Flickr. I use Aperture, and the latest version includes easy publishing to Flickr. This built-in "publication" utility allows for subsequent synchronization of published albums when images are modified or metadata is updated. I understand that iPhoto features a similar built-in utility.
AppleTV accesses Flickr accounts so that one can display images and slideshows on an HDTV via the web.
Why has Apple so blessed the Flickr photosharing service while snubbing all the others? I don't know. Am I happy about it? No. Is it trivial to move thousands of photos from one service to another? Not at all.
I'm dealing with the reality imposed upon me. At the other end of this labor, I'll have online albums that can easily be updated and that can be seen on anyone's HDTV via AppleTV. Most of my extant SmugMug albums are now up. There's still a bit of work to do (especially with Rio Phyz albums), but most things are in place.
The SmugMug albums at phyz.smugmug.com will remain online until my subscription runs out in October. The Flickr albums can be accessed via flickr.com/phyz, but I prefer the "Collections" page as a starting point.