I've been using dual monitor setups at home and at work for a few years now through several hardware builds, both desktops and laptops first using Ubuntu, then switching to Debian Testing for a rolling distribution about a year ago (after finally realizing I love the latest software, but don't want to spend time to upgrade/rebuild every 6 months to keep up with Ubuntu's release schedule or deal with the hassle of installing everything from source). I've been using Nvidia cards with the nvidia driver and dual monitor support has been fantastic for me.

Well, I've been using Nvidia cards with the nvidia-driver and dual-monitor support as well, and it has been fantastic, until I upgraded my Ubuntu install, got Unity without asking for it, after which multi-monitor support was completely broken. My colleague who has 2 screens of a slightly different type (all are Lenovo ThinkVision) and is running Arch with Gnome 3, has been experiencing random multi-monitor glitches since day one. Sometimes for no apparent reason one of his displays doesn't get a signal after waking his laptop from sleep or hibernate, and the only way to resolve it is a reboot.

To make a long story short: we could exchange anecdotes all day about the state of 'Just Works' on Linux, but at the end of the day, I think no one with enough experience using various Linux distros and OS X, can honestly and sincerely say Linux is even close to OS X in that aspect.

Myself, I've been using Linux since Slackware 4 and have tried about 10 different distro's over time, alongside OS X for the last 5 years or so. Up to this day, I regularly run into problems that need fixing on Linux, particularly after upgrades, or when switching hardware. Whether it's Wifi cards, USB hardware, multi-monitor support, network configuration issues, software that stops working, system library problems: there's always something. OS X on 3 different machines, from OS X 10.4 through 10.7, I've only had one issue that required maintenance once, on a b0rked upgrade. It was pretty nasty, but fortunately OS X has Time Machine and target disk mode, so in no-time I was able to pull off any important data just to be sure, re-install the OS, restore my Time Machine backup, only to find out everything was back to normal, to the point I didn't even need the files I had to pull before the restore.

I still haven't figured out how to get my phone to tether over USB on my friend's Mac. Works flawlessly on my Debian machine though. It's not like OS X is seamless.