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I've been a big Linux user for over 3 years, and I've been very interested in trying BSD. My only foray into FreeBSD so far has been the FreeSbie live CD. However, I've been following BSD for quite a while, and have been considering doing a full install, but have been scared away by various press releases, reviews, and user comments in message boards (including those coming from BSD users themselves). My impression might be completely wrong, or misguided, since I have very little hands on experience with the BSDs. But here are my impressions:
FreeBSD - Well integrated kernel and userland. Supposed technical superiority. Efficient and stable, at least with the 4.x series. 5.x series has had major bugs/problems, which seemingly have not been fully fixed. When a major dev decides to fork because of the problems (to DragonFly), and when a user who likes BSD says it's as unstable as Win95, and when it gets lots of bad press, I decide to say "thanks, but no thanks". Major problems with infighting and politics within dev community also exist - I have no desire to start using a system that has internal politics amongst it's devs getting in the way of fixing problems and moving forward.
NetBSD - super portable, but don't know how useful, how stable, how fast, or how technically excellent it is. It claims to be very technically pure and simple, as a technical strategy for achieving it's exreme portability.
OpenBSD - super secure, technically excellent, pure code, super stable. It's major use has been as firewalls, and some web/file servers. Some have said it is slow. Installation is a pain (from just looking at the installation documentation - it looks absolutely horrible). No problems with internal politics because has central control from one individual (a benign dictator).
Thus, I just don't see, yet, the possible benefits of using BSD (other than OpenBSD if I wanted to lock things down in a network, or NetBSD if I wanted something to install on some obscure hardware).
As for technical excellence, speed, security, and stability, Debian and derivatives have that in spades, Slackware, too, is fantastic with speed and stability and technical excellence (if a bit more work to get installed and configured). Sure, some Linux distros have been slow, bloated, and buggy, and not so technically excellent (FC comes to mind). But if you stick with Debian (and some deriviatives) or Slackware, you will have a technically superior system.
It seems to me that FreeBSD does not have any major overall superiority to a well-designed Linux distribution, although each had advantages in certain areas. Slackware is possibly my favorite Linux, and moving between Slackware and FreeBSD is not a very painful thing. I have *definitely* seen major issues with Fedora Core 3 in comparison to FreeBSD, to the point that a major statistical application (PostgreSQL, R, PL/PGSQL) some friends of mine wrote runs perfectly on a very low-end FreeBSD 5.4 box, and falls apart in minutes on a much bigger FC3 system.
And for all those complaints about FreeBSD 5 being a disaster, remember that the vast majority of that was in 5.2 and previous versions, which were NOT recommended for production anyway. And Linux had its own history of problems when the 2.6 kernel began moving into production, so let's not start trading barbs, please.
Anyway, what we FreeBSD users mostly love is not any specific (perceived) technical superiority, but the integration between kernel, userland, and ports. With FreeBSD, everything moves together, making it much easier to manage a system overall than any other OS I have dealt with. Makes it much easier to run a clean, stripped-down system while still staying up-to-date with any level of your system. And before anyone starts ranting about source-based updates, search and you will faind that you that you can mix binary package management with source-based management all you want. You can get binary kernel and userland updates too, but I much prefer to compile my own, and FreeBSD makes it so easy to do that this is hardly a disadvantage. (that's right; it is much less tedious to compile the FreeBSD kernel than the Linux kernel)