The bugs that I remember being described in 5.6 are:

• Some odd regexp bugs caused by the new RE engine. The situations they cropped up in seemed to be rare, but a few people (5ish?) reported running into them.
• CPAN modules with XS components, that had already been installed, sometimes wouldn't work. I believe the solution was to rebuild the module or to rebuild Perl with an option. I can't remember which.
• Unicode support is only half-there for string operations (which shouldn't break much, seeing as how there was no Unicode support in 5.005 :-)
• The lexical warnings stuff never seemed to work for me ("no warnings" didn't stop warnings).

My advice is to stay at 5.005_03 until 5.6.1 comes out, simply because installing a new version of Perl *always* is a pain in the butt, even when it goes smoothly. It's work you don't want to have to do twice, a month apart.

5.6 has a whole lot of sexy new features:

• Autovivifying file handles. No more screwing with typeglobs, if you want to open a filehandle and store the filehandle in a scalar:

  open my $fh, "< foobar" or die;

• 3-arg open to separate file mode from filename:

  open my $fh, "<", $filename or die;

• POSIX character classes, and extensions to the POSIX character classes.
• Unicode everywhere. Coolest is \N in double quoted strings for named characters: "I \N{heavy-black-heart} Perl" and Unicode character properties.
• our() keyword, a lexically-scoped version of "use vars".
• version strings, a way to make binary strings that can hold byte tuples:

  $str = v3.14;
  $str = 12.1.128.19;   # v not needed if multiple dots
  [download]

• threading is damn near usable (everything but $1 et al works with threads, I believe). People have actually been using it.
• compiler is closer, but still needs work.
• BEGIN and END have friends, CHECK and INIT. CHECK blocks are done as last step of compilation phase. INIT blocks are done as first step of execution phase.
• warnings module lets you enable and disable warnings for a block, also lets your module specify categories for the warnings it issues.
• lvalue subroutines:

  foo() = 5;  print foo();
  foo() *= 2;  print foo();
  [download]

  It's a subroutine that returns a value, but you can change that value!

I wrote the "what's new in 5.6" section of a new class, "Advanced Topics In Perl", so it's all fresh in my mind :)

Cheers;

Nat