I've upgraded my mother's Mini from 10.4 to 10.5.7 and installed a VisiKey keyboard.
I made the move to 10.5 because 10.4 is nearing end of life, and I thought 10.5 was becoming reasonably well baked (wrong, wrong). I also wanted the option of using 10.5's mediocre iChat in place of Google's elder unfriendly Google Video Chat. Lastly, since I no longer have a 10.4 machine at home and I use LogMeIn to manage her machine I wanted her on the same OS as our family.
I installed the VisiKey because her macular degeneration has progressed far enough that the need to see the keys has overcome her fondness for the cool look of her Apple keyboard (she's not a geek, she really does like "coolness").
The VisiKey's not bad, but there's a bug in the driver installer. In a multi-user machine you have to manually add the VisiKey driver to each user's LogIn list. Without the driver most of the kb features work, but not the "Internet", Email, and Search buttons.
Although I'm no great fan of 10.5 (and believe me, I'm going to take my time with 10.6!), there are a few accessibility improvements. In several app toolbars (Safari 4beta, Mail.app 3.0) there are Smaller/Bigger buttons like these (grayed out here so very murky):
Ok, so I lied. They're not automatically there, you have to customize the toolbars to get them (right click on toolbar then choose customize). You have to modify the toolbars in Mail.app for browsing, new message, reply, etc. In some cases, like "New message" you can add these controls but they don't seem to do anything . In reading mode, however, they do work .
I also found the album view in iTunes 8.1.1 isn't bad for low vision use.
So a few accessibility improvements, though so far they don't outweigh the misery of 10.4 to 10.5 migration for me.
Update 5/31/09: There are bugs here. In some modes they enlarge all text, in other modes you have to select the text first. Looks like the responsible dev teams weren't always on speaking terms.