It's been about four-and-a-half years since I bought the last one. I'd been hanging on for Apple to release a machine with the latest quad-core Intel processors, but those got released just before these new models, back in June, and so I am stuck with some earlier year's. (I think Apple might have known that the yields were poor.) What forced my hand was that Isabelle started reliably crashing the old machine, perhaps due to the heat, dust and possibly borderline support for the memory I put in it. Or maybe it did have the infamous GPU hardware bug, despite getting a new mainboard in September 2013.
Anyway, what I give up with the new machine is a DVD drive, which I only ever used to reinstall the OS, and an ethernet port, which I will miss if I ever get back to hacking hardware. Also they have gotten rid of the Kensington lock slot. I now have to use Yosemite, which is not hugely different to Mavericks, so shrug. What I gain is USB3, a superfast SSD, a very sharp screen, and less weight to lug around. I doubt the battery life is going to matter, or the extra ThunderBolt port. So faster, yes, but otherwise pretty much a wash. I do like having a machine where the only moving parts are the keys and the fans though.
Buying the thing was an ordeal. It was quite expensive: $2.5k and another $230 in Illinois state sales tax (I mind paying money to the state less than to the Fed), but amortised over four years it's not so bad ($2 a day). I live quite close to the Lincoln Park Apple Store and figured that if I put enough cash on my Visa debit card, things would go OK. But when I got there I figured what the hell, let's try putting some of it on the credit card. This failed, and the card got totally blocked. They like doing that to me — why they can't just reject the transaction and notify me I don't know. I'm never going to rely on it when I'm overseas. Then the debit card got blocked too, as it has a $2k daily limit. The lady at my credit union told me about that, and said I could go pull another $1k in cash from an ATM. Another two calls to them got it unblocked, two trips to the ATM ($600 limit per withdrawal there) got me the difference, and an intervening switch of sales assistant finally allowed me to pay for it. The machine was brought up from the stockroom three times, by the same girl, to my excruciating embarrassment. They insisted I take my 84 cents in change. This is why I shop online.
After that I spent the whole afternoon sitting in the Apple store reinstalling Yosemite (all for a case-sensitive root partition) and Xcode. Everything comes off the internet now, so it's slow. At home the Migration Assistant took five or more hours to scrape my data off the Time Machine backup. After that, reviving the usual arcana (MacPorts, the venerable perl blogging script, some settings) went far more smoothly than previous times.
Update 2015-08-02: The right shift key ceased to pop back up after mild use. A trip to the Genius Bar at Apple Lincoln Park on Monday 2015-08-03 got it replaced, and we'll see if it's fixed.