I've borrowed a MacBook Air for a week and decided to share my thoughts as I look back.
Why? A great magnetic 1TB disk in my premium Dell XPS 15 got borked after 1 year of its loud and hot service, so I had to replace it. I also kinda sorta have to code every day, so I had to borrow something.
I decided to borrow a MacBook because it has the reputation of being a "it just works" kind of machine, and since I had to have a working environment as soon as possible, it seemed like a great idea.
The tiger or whatever
The OS had a brief wow effect on me, because everything looked gorgeous. I also found consistency of notification sounds across applications pretty surprising and pretty cool. That's the privilege you don't get on Linux.
Being able to run shitty Safari natively was also something I enjoyed a lot! I would sometimes just open Safari when I was bored to see it come up and kill it immediately afterwards, like any sane person would do. I had such great time.
Something was pretty obvious from the start, though. The trackpad is a pretty serious business in the Apple world, apparently. These guys don't mess around with their cursors. Prepare plenty of your fingers for some exercise!
As a Linux elitist and a terminal addict that I am, I find that to be annoying. I use Openbox as my window manager for a year now and I'm enjoying it so much, because it gives my keyboard so much power, a power that is being neglected in OSX. I can do everything with keyboard, from standard hotkeys for opening applications to window movement, resizing and layouting. That's why I just hate mice and their successors - I might be biased though, since I own two cats.
Installing applications confused me a lot too.
The Dock is another annoyance. I don't understand the reasoning behind using it on a screen with a max resolution of 1366x768px, where every pixel matters. It's also remarkably bloated with, let's face it Apple, crap no one uses by default. Trying to kill it, like any sane person would, revealed another hidden gem of OSX - the famous iNability. It manifests itself in preventing you from doing something. Being a Linux user, obsessive about configuring anything that you could possibly think of, that's something I'm definitely not used too. As this StackExchange answer shows, there are some workarounds but they are really lame. I don't know for you, but I had some trouble sleeping knowing the dock was still alive and well.
In the brief period of my usage of OSX, I've managed to discover another iNability, without really working hard to find it. I like to use just the external monitor so yeah, I like the internal monitor powered down, thank you very much. You apparently can't do that without closing the lid, which makes typing pretty difficult. The workaround people use is to just minimize brightness and go on with their lives. It's true, they don't even care if their internal monitor is still consuming power or not. They don't even care about their GPU still having to render stuff on both monitors. It's crazy sometimes, but yeah - I don't even words.
To MacBook or not to
I'm pretty sure I'll never leave Arch Linux and even replacing it with another distro would be somewhat weird for me, let alone replacing it with another OS. Sure there are some benefits, like running certain apps and browsers without VMs and and the more polished look of the UI, but the cons outweight them.
I don't want to sacrifice the keyboard and insane customizability just for the flashy looks. There is no you can't in Linux.
Speaking of hardware, MacBook Air is a great machine, no doubt about it, I regret buying this Dell instead of it, since Air costs less and is better at everything except CPU, but meh - i5 instead of i7 so it's not a deal breaker. I'd exchange my Dell for Air in no time.
EDIT: I'm now using a Macbook Air with OSX as my primary machine and I'm really happy with it.