My primary computer is a 15" 2012 MBP, the last and IMO best of the expandable/upgradable laptops. The beauty of those is that you could buy a somewhat base model, and the only thing that really locks you in is the CPU(and VRAM on dGPU models). I have 16gb RAM, a 1tb SSD, and even a 2tb spinner for tons of storage space(the SSD is the boot drive and is in the optical bay, while the spinner is in the standard bay-I have it set up that way because the optical bay lacks the SMS of the main bay).
I also use a 2010 Mac Pro that really is a beast, or at least I consider it one. It's dual hex cores with a base clock of 3.46ghz, and I have 88gb RAM in it(I bought 96gb, but got a dead stick and the seller claimed I killed it because I put "server" memory in a desktop, but that's beside the point). It boots off PCIe SSDs, with some decent sized spinners in it for storage and also internal Time Machine drives. I had USB 3.0 in it at one time, but took the card out since it causes some weird interference that makes Bluetooth not work. The only real weakness on mine is the GPU, but primarily because I have some tasks where I need Snow Leopard/Rosetta and to integrate it with newer Intel-native software(and virtualization won't work) and the Radeon 5770/5870 are about the best I can do to use a somewhat current OS(HiSi) and SL.
I have Macs from 1984 that still work. Are they useable? That's not a straight forward question, as the original 128K is a handicapped computer. I have a Plus with a 1984 128K front bucket/CRT but is a lot more usable thanks to the fact that the Plus is just a much better/more polished computer(back in those days, Apple often offered upgrades to newer models when the case didn't change appreciably-the 128K/512K to Plus upgrade replaced the logic board, floppy drive, and rear bucket-the last for the different port layout).
In there, I have a bunch of different ages of laptops. Apple has made some wonderful laptops over the years, and some not so great ones. The Pismo G3s are particularly durable, as are most of the iBooks. I've had bad luck with 5300s cracking, and some super low end models like the 1400c and the first generation Wallstreet "Mainstreet" PowerBook(without L2 cache) are painful to use even with software that would have been current when they were new. It's mostly hard to beat a Titanium PowerBook for an OS 9 laptop as long as you don't kill the hinges. The first 17" aluminum PowerBook can be coaxed to run OS 9 and does so quite well, but the L3 cache has a nasty habit of dieing.
With all of that said, there's a difference between "still works" and "is useable." Up until a few years ago, I often recommended late '07/early '08 MBPs(as long as they had the replaced good GPU, which can be done now by a few folks, and Apple finally figured out by the end of the warranty period) as great starter Macs, and I was perfectly content using one even in 2015. Official OS support rolled off El Capitan, and the late 2007 can not be made to run anything newer(the early 08 is a different story).
I have a 2011 17" sitting right next to me that I really need to get sent to DosDude to have the dGPU disabled. Unfortunately, dGPU failure on 2011 15" and 17" computers is not a question of if, but when. This nearly unused 17" lasted a couple of hours after I got it. On the other hand, I'd not hesitate to pull out my 2011 13" and live with it for a while if need be aside from the fact that it needs a new battery. SSDs make a huge difference on all of these.