The disadvantages of iPad 1 now:
Limited RAM (new iPads have four times as much).
No ability to get the latest firmware.
Why you should care:
Limited RAM can lead to repeated crashes, depending on which apps you use.
No latest firmware means you can't load the latest version of some apps.
Right now, Skype and Netflix require iOS 5, which is the highest firmware that iPad 1 can take. It's up to developers how long they offer support for older versions; the later the iPad, the longer support probably will be offered.
Between my husband and me, we owned three iPad 1s, which we've given away. If we hadn't, we'd still use them for limited but diminished uses, like as a photo frame.
I wouldn't buy any iPad 1 now. The lowest I'd go, if budget didn't afford a new model, is iPad 2. At least Apple is still selling them and iPad 2 shares some of the old guts of iPad mini, which is a relatively new device. With iPad 2, there will still be RAM limits, which can lead to crashes. iPad 2 has half the RAM of iPad 3 and 4.
Increasingly, developers are upgrading apps to take advantage of newer hardware -- faster chips, more RAM, retina screen. That will increasingly limit iPad 1's uses and reliability. (I saw decline on iPad 1, which I owned from debut day of the 3G model, as compared to iPad 2 and 4, which I still own.)
By the way, the Facebook app seems to crash a lot, from various reports on the forum, esp on older iPads.
