If you have the navigation bar enabled on your journal, you'll see a bar beneath the navigation bar that says: 'You're using the old version of the Friends page — switch to the new one.' Clicking the link will indeed allow you to test drive the new page and see what it looks and feels like.
You can easily go back to your standard flist by clicking go back to the old flist link that will appear on the proposed new page.
If you haven't got the navigation bar enabled and would like to see the change clicking on the above 'switch to the new one' may work (it does for me that may be because I'm me using it). Or you can go to customise your LJ theme and turn it on from there - and do the same to turn it off again.
I tried it out for my first daily trawl around LJ (and once I'd done that I switched back to my nice, shiny, flist that I love and can actually read easily and has the functionality I like). Personally whilst I do not like it (er. make that hate it), find it very difficult to read, miss my nice looking flist and the links and side-bar, hate the infinite scrolling (even though it isn't a problem technically for me) it isn't quite as bad as I feared. I don't want it to stay and I don't see any point in introducing it (once again they are trying to fix something that isn't broken). After all, one of the biggest advantages LJ has over other blogging sites is that we can have our flist looking as we like it to look.
And it's not just a case of look, but of functionality - I know a lot of people will have major problems with the infinite scrolling and it will keep crashing their browsers. Plus, the white space will cause major problems for other users. This is why people like to customise their LJ to choose a colour and font that works for them, not just one that looks pretty, but is actually readable, doesn't give them headaches or cause eye-strain or make it impossible to read, reasons that are very important.
The new design and beta testing has been announced in lj_releases - not a comm everyone reads. Comments on the new layout can be left on the post here.