In the Windows and Linux versions, you can go to the URL by pressing Alt and entering the number of the listing, or you can click the desired result with your mouse. Other useful commands are "define" to get dictionary definitions, "map" to find a specific address or city in Google maps, and there are many more. Entering "command-list" will bring up the latest list of available commands.
An interesting feature is "translate." One can highlight a portion of text, open Ubiquity, and type in "translate this to french" or "tra this to french" to have the French translation set directly into the highlighted portion. This works very well and very quickly.
"tweet" or "twitter" plus text sends your text to Twitter. "lookup," "wiki," or "wikipedia" searches Wikipedia for the terms you specify, as does "msn" for searching MSN. "yahoo-search" and "youtube" work similarly. "movie" or "imdb" searches the Internet Movie Database website.
Overall, Ubiquity is useful now although it is far from a finished product. It's likely that a Ubiquity-like feature is going to be included in a future version of Firefox, so if you'd like to get a feel for it now, there's no reason not to try it - as long as you are aware of the limitations of this alpha version.