Works really well so far! I'm playing on a 2007 iMac w/ 4GB memory. Crappy ATI video card. It's a little slower than how it runs on my beefier PC, but not by much.
I had SA installed on a networked PC, so installation was really quick. Didn't test downloading the entire client.
1) Download the SA installer (http://www.uoherald.com/trial/)
2) Create a Win XP Bottle
3) Select the Bottle and go to Advanced menu, choose Open C: Drive
4) Create the directory C:\Program Files\Electronic Arts\Ultima Online Stygian Abyss
***If you do not create the folders to make that path exist, the installer will fail
5) Install New Software - Choose the SA Installer
6) Install DirectX when prompted, choose default values for the rest of the UOSA installer
7) Finish Installation
Either PATCH or COPY the game files. I chose to COPY:
1) Open your external source (networked PC, USB drive, etc)
2) In Manage Bottles, open the C: Drive in Finder
3) Copy contents of UO folder in external source to your new install. Replace all files.
I don't want to play in full screen mode and found that windowed mode freaked out when you try to resize. To fix this:
1) In WineCFG, emulate a virtual desktop. Choose the size of the window you want to play in using the resolution settings.
2) Start UO, and go into User Settings and set to play at Fullscreen at the resolution you just picked.