Well, there is NO such thing as VLC "Beta Version" and I doubt there ever will be any. Nightly builds are regularly released every night (duh) and they are automatically generated, so there *might* be errors.
Current version, 0.8.6i +is re+co+mmended +if you want it for any critical applications.
But its a BOON for many if you use 0.9 svn. Because it runs away from the old wxWidgets based interface. This new baby has an interface written in QT4 and its quite stable by now.
On linux, I recommend you try out the nightly repositories MAINLY if you use KDE since QT4 intergrates awssomly into KDE. Its quite buggy, but you will enjoy discovering them +and r+e+porting them. Tru+st me. YJ+u+st++ ++r+e+por+t in thei+r forum.
On windows, well, QT4 interface is still under heavy development and its still ugly IMO. The best is yet to come, and as always, windows users are adviced to get only the stable versions. So go ahead and get VLC 0.8.6i.
On mac, VLC is written in Cocoa. Aayush, Milind, Sunny, Manan and other mac users would like this. I doubt its going to change since cocoa is DESIGNED for mac. So they too should get the latest stable release of VLC 0.8.6 mac-cocoa DMG.
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O+ther Differnces:
The m+ain+ t+hing the new versions have is fixes+. Several+ issue+s a+re fixed.+ +VLC can now fore+w+ard in+ FLV fi+les+, h264 rendering is MUCH better +and mu+lti+core +su+pport for the same is bei+gn wo+r+ked on.++++
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Final Advice++'++
+
Get the stable +rele++as+e+. If+ something is not satisfactory+ or if its buggy, get the nightly buil+d +++++++++++++++++++of the night you are in+ and see ho+w+ it w+orks. If you are satisfied w+ith it, keep it. Else, check out next night.