I've been using UAE4ALL on my Dreamcast since it came out.
In a nutshell, it blows. Big time.
It's somewhat slow compare to an A500. And don't even think about using Workbench. I made an ADF of my old Workbench 1.3 floppy and loaded up on UAE4ALL, and about 20 seconds after I started using it, the mouse screws up. This happened every time I loaded it up.
All in all, I've been waiting for a newer version to come out.
As for the ST emulator, it's awesome (but I still love Amiga, OK?). 100% speed and sound, the latest version fixed the contracts and clolrs and it loads Speedball 2!!!