1.) Quality and Value. I believe Spyderco to produce the absolute best bang for your buck. The fit and finish, especially of the pieces coming out of Golden and Taichung, is regularly better then any other full production knife company (in my opinion). Many of their offerings come with premium materials, and yet, the price is very reasonable. I regularly recommend the Sage 1 to people getting seriously into knives. S30V, carbon fiber, flawless construction, for about (or less than) $100.
2.) Design. Spyderco are the masters of producing knives that just feel right in the hand. They are generally known as the ergonomic knife company. When I got my first (a Sage 1), I think I audibly said "wow" to how it just fit into my grip so naturally. Also, Spyderco's designs are original. Though they do work with other knife designers on some pieces, their overall design aesthetic is completely their own and unmistakable. From time to time we hear about certain companies and their questionable ethics, and how they ripped off this or that design. I have never heard that from Spyderco, though they have been on the other end of that.
3.) Great Service. The customer service you can expect when you buy a Spyderco is superior to other full production companies in my opinion. They are extremely responsive and will work with you to correct any issue you may have.
4.) They're knife guys. They aren't a company that exists to sell you knives. They live knives. Sal Glesser is the most responsive owner out there. He regularly shows up on a number of different knife forums. I once saw in a thread (sorry can't find the source), where someone was complaining about an issue with their knife, and out of the blue Sal shows up and asks the guy to send him the knife so he can look at it. Furthermore, I love the Mule concept. Spyderco regularly brings in samples of many different types of steels available to evaluate them, and while they are at it, they produce Mules (blank fixed blades) for sale at a reasonable price so that we can all try them out too.
You know what, I feel like I'm gushing, just go buy one.