I've seen so many people state the same claim you make - The Perdix/Teric is the best computer.... Why is it? Now don't get me wrong, they are great tools, and shearwater have done a great job making and marketing them, but what are the criteria they're being objectively measured against? Or is it a collective subjective opinion?
Well, to be fair, I haven't tried every other dive computer. The reason I think that Shearwater is awesome and worth it for any diver if you can afford it has nothing to do with their technical diving capabilities.
1) Build like a tank. VERY reliable gear, and I have used and abused a few.
2) Incredibly readable screen (admittedly many other brands have this, but Shearwater stands out).
3) User replaceable AA battery (not the Teric, obviously). This has saved me on many occasions. You can get a AA battery anywhere in the world, and you can replace it easily on a bouncing wet dive boat.
4) Great user interface (I hate those one button computers).
5) Will grow with you. Yes, if you will never do tech, this isn't important, but it also means that Shearwaters really hold their value for resale in the future - you can sell it to any diver.
6) Very reliable download system (I struggled with Suunto cables for years).
7) Great information on their website (you don't need to buy one to get this). So many terrific articles about diving physiology, etc...
8) The most important one. The company provides AMAZING customer support. Whether you need to send it in or just need help trouble shooting something, they are there for you. Knowledgeable, available and helpful. I have on two occasions emailed in a question on a Sunday night and gotten a response within less than a half an hour.