ok!  i finally have time to reply.

by usable i mean that people visiting find it easy to use.

for instance, i love Flash and have used it since version 3 or 4 and ActionScript 1.  i'm also very familiar with its drawbacks.  you need visitors to have a specific version of the Flash player, anything in ActionScript 2.0 and lower generally didn't play well on a Mac (it was a resource hog), and just in general, lots of people hate it with a passion.  Google only recently acquired the means to crawl it.  no one can bookmark a specific piece, and back and forward in the browser not only don't work, but if someone doesn't think and presses back, they will be forced to navigate to the same place they left if they want to continue on the same path.

an all Flash site can be very useful when you want to set a mood or you are serving a specific audience it will impress.   or incorporating true interactivity (which means visitors can actually do something they enjoy with the site, not just click a button and have something happen).  or if you're just making a site for fun (a vanity site).   it is not great if what you do is still illustration and all you did with it was make stuff fade in and out and slide in and out.  in return for pretty trivial presentation advantages, you lost your ability to be found, created some hostility to your site and yourself, and generally made it harder for people to get to your work.  and that's assuming you created an interface in Flash that's as usable as your standard HTML nav bar, which is uncommon in portfolio sites. 

Bard Computer Science - not a portolio, but a good example of a richly designed interface that isn't Flash
N.Design Studio
sThig Illustration - iirc, this artist actually starts with Poser renders for reference.
digital art by henning ludvigsen - the interface is a bit hard to read, but the portfolio has a good structure.  each image has notes, comments, a detail image and a process image.  it's a great example of a portfolio meant to establish the owner in the community.  it's also a creative use of Wordpress.