I didn't have too much time, but tried it out during a 5 minute lull at work on my Sony Ericsson X1:
* First, it wouldn't run at all: the X1's screen is WVGA (800x480) and Skyfire only supports up to VGA (640x480).
* Using a WVGA hack from xda-developers, I was able to start the program up with 160 pixels of the screen staying black.
* Page load times were relatively quick and overall, websites looked pretty good, essentially identical to what you get on desktop.
* The UI of skyfire is decent, roughly equivalent to Opera, although still far behind Safari.
* From a quick test, Flash did indeed work on Youtube. The video seemed smooth, though I didn't watch for long.
* The one issue I saw - probably just a bug in this beta - is that occasionally, pages just wouldn't load on the first or second try. For example, you'd type in cnn.com, push enter, it would start loading... and then nothing would happen. Click on a youtube link, the progress bar would start filling up... but nothing would appear. After one or two retries, it would always work, but it's clearly an issue.