Getting to try SL in a browser wasn't as easy as it seems. When I read the beta is open, I went to http://interest.secondlife.com/beta and meh - I wasn't qualified to test it.
(thank you, SignpostMarv Martin!)
It works fast. Really fast. I'm impressed with Gaikai technology. Everything rez in time, the teleports are working properly, you can chat with other avatars - just as if you were logged in a regular viewer. It does have some limitations though: you don't have your inventory, your look is limited to pre-defined avatars, you can only teleport to locations from 'Destination Guide' and your name is simply Guest.
The controls and options you have are limited - generally, the web viewer can be used to explore and chat only. But, the overall experience is fun and you have to keep in mind this is only a beta, so things might change in future. If Linden Lab will continue to pursue the idea, that is. It's interesting though: imagine embedding a location from SL on your website, so that visitors can log in to your events, meet with SL residents at your venue, and maybe register for regular Second Life if they like the experience, since SL sends a 'There's more to explore!' e-mail after each use of SL Web Viewer.
I'm certainly hoping to hear more on the topic of web based SL in future.