Original iPad owner joining...
I purchased the 12th iPad 1 sold in Ann Arbor, MI back in spring of 2010. It is now over 3 years old and I still use it. I have not updated by buying the iPad 2 (well, actually I bought two, but as gifts for others), or the iPad 3 (well I bought one for a gift for another), the iPad 4 (really no). I did just purchase an iPad Mini with 4G LTE and Product Red case, which I love, but it will never replace my Original iPad.
One reason I so like the iPad 1 (in addition to the sturdiness mentioned by others) is the Apple keyboard dock, which only fits it (not the more recent iPads). It is no longer offered by Apple, but it gives you the great Apple keyboard and a mount to hold the iPad handy as a touch screen display. Other iPads are too thin for its connection.
(I tried to post a picture of this great keyboard dock, but the forum software says I cannot post links or images until I have over 3 posts... seems counter to the idea of minimizing post count while maximizing info content! Do a search for Apple iPad keyboard dock to find a picture).
Next iPad will be the iPad 5, as I do like the Retina display and will like its purported 25% lighter, 15% thinner than the iPad 3/4.