Ignore Canar's sarcasm. Everything you need to know is in the change log at http://www.foobar2000.org/changelog.html

As you will see, version 0.9.0 "improved security by replacing various third party libraries with own implementations," and 0.9.3 included a security fix in the Ogg Vorbis library. IMHO it's a bit melodramatic to say 0.8.3 is "insecure", if the Vorbis bug is the only known exploit risk.

Also, calling it "buggy" is a matter of perspective; sure, there are bugs that you may or may not have encountered, and some of them have indeed been fixed in newer versions. The current version has bugs, too, though. The question is whether you're negatively impacted by any of them. Obviously you're not suffering too much from the bugs because otherwise you'd have sought remedy.

0.9.5 does introduce many UI changes, but they are mostly cosmetic or nice-to-haves, nothing essential. 0.9.4.4 introduced support for Musepack files. I found this to be helpful when I needed to play one. Most of the other 0.9.x changes are miscellaneous bug fixes (including a few application crash scenarios) and feature bloat. Nothing is really crucial, if 0.8.3 is meeting your needs.

Inexplicably, it's the last item in the 0.9.5 changes that's the kicker for some folks: "Windows XP or newer is now required" ... the app will not run on Win2K anymore. There's a forum post here that explains why. Inexplicably, old versions aren't available for download, and no info is given about where to find them. I guess when they say they've dropped Win2K support, they really mean it! As mentioned a couple weeks ago here, to get a version compatible with Windows 2000 (I recommend 0.9.4.5), you need to rely on a third-party site: http://www.filehippo.com/download_foobar2000/