There's a beta out. http://rarlab.com/rarnew.htm
The main thing to note about it is the new Rar5 format, which only WinRAR 5 can create and extract from (expect third party support for decompressing the format later on).

So if you get any weird RAR files you can't extract from in the future, this could be the reason.  Or if you do decide to get WinRAR 5, you may need to ensure compatibility with older WinRAR users.


Now the geeky stuff.
It seems that they've increased the max dictionary size to 1GB, which possibly erases most of the benefits in compression that 7z had over RAR (RAR was previously limited to 4MB dictionaries, whilst LZMA could go up to 1GB).  Changelog also states better multi-threading support, so that may solve some speed issues.
They have seemingly removed PPM compression, which may mean that text content or file sets with WAV/BMP files or similar may not compress as well.
Large dictionaries do require more RAM though, in fact, LZMA using a 1GB dictionary requires around 10.8GB of RAM for up to 3 threads.  Dunno how WinRAR scales with this memory usage.
Besides that, Rar5 may close the gap on compression performance compared to 7z.  Haven't done any tests.

Though 7z is open source, so I'm still going to support it always :P

Peazip seems nice, but unfortunately was rather buggy when I tried it :/

xz, I believe, is just LZMA2

The main problem with the .tar.*z formats is that it's an entire archive wrapped in a compression stream.  This means that you need to decompress the entire thing before you can do anything useful to it (even viewing a file listing).
It does guarantee solid compression though, and keeps Unix file properties.