Questions on Slack / *BSDs

[Sven Burgener <svenb@bluewin.ch>]

Dear debs

Could anyone using (or having used) Slackware please tell me what's 
(particularly) good about it? -> What particular things made you 
choose Slack?

>From what I've read, it's probably the distro closest to the
"roll-your-own" type of thing. Correct?

Slack's package format is ".tgz"; correct? Is that a plain tar file, like
the ending suggests or what?

Now, whilst we're at it; I'd also like to hear what's special about 
(Net|Free|Open)BSD.

Firstly, what are the differences between them all?
And, how do they compare to (Debian GNU /) Linux ... and to each other?

Which are free? (Suppose OpenBSD sounds pretty free :)

I've never used anything but {Debian GNU,SuSE} / Linux and AIX.
(Talking about real OS' here, so I'm not counting winhose :)

Thanks
Sven