I love mine, but I use it mostly for "recreational computing" and not work. I'm typing this on it right now, in fact. I write code for a living, and while I can write code on this, I prefer larger, multi-monitor setups when I'm actually working. If it were my only computer, and I tried plugging it into a bigger monitor, it might be okay, but it is by no means a beefy machine, and I'd probably get a 13" rMBP if that were the case.
I'm not a tablet guy. I have had iPads at various times and it's just not enough for me. I can't use them for an extended period, can't type without an external keyboard effectively, and can't use all the software I need. I have a 6+ which has a nice big screen but is also pocketable, but that's not quite enough screen when you're lying on the couch or in bed and want to watch some Netflix. The MacBook is perfect in this regard. It's basically the size of a tablet but gives you a full-fledged OS (plus you can easily open the display to whatever angle you want, rather than trying to uncomfortably make an iPad work when in bed). My wife can use it too since you can have multiple users on OS X, unlike the iPad. The screen is gorgeous and you get used to typing on the keyboard. I use big beefy mechanical keyboards when I'm working, so it's nowhere near that, but I actually like typing on it better than my work 15" rMBP.
The MB fits perfectly into how I use devices. I was using my work laptop as my personal one, and it's a bit too big and full of my work crap to really enjoy using on the couch while watching TV with my wife. The MB is also obviously great for travel. I took it on a two week vacation and it was great. Ridiculously light, good battery, let us do all the Internetting we needed to do while away.
I have a 15" rMBP for work and a 5k iMac for my personal heavy-duty computing needs. If I didn't have those I couldn't live entirely off the MacBook. It fills a gap for me that I've been waiting literally years for (I wasn't going to buy a MB Air because the non-retina screen would bug the shit out of me). I guess the way I use it is kind of an iPad on steroids, but for me that's what I've been craving for a long time. I don't care about the lack of ports (I use it when I'm laying on the couch or traveling, so I don't want to have a bunch of shit hooked up to it anyway). I just want a lightweight laptop that lets me check my mail, watch some videos, or play a simple game (lower-end Steam games like Prison Architect and Papers Please run fine). For these things it is perfect.
It's expensive for what it is, but we're all Apple fans here, and if we weren't willing to pay a premium for that then surely there are cheaper alternatives. This thing fits into the ecosystem very well. For casual computer users it would be fine, too, I think. It's so insanely tiny that you really can overlook a lot of its shortcomings. I thought my rMBP was pretty light, but it feels monstrous now after having used the MB.