In my case, the iPad runs on iOS, a solid platform that constantly keeps improving.
Back in 2010 when all kinds of tablet were announced, I didn't buy an iPad until June, mainly because I was waiting for all those other incredible tablets to come out. The wait was for nothing; these tablets were all abandoned by their manufacturers and all their amazing promises were never delivered, so I got tired of waiting and finally decided to get an iPad on the last day AT&T offered the Unlimited Plan for the iPad.
Now that I new tablets came out like the Xoom and the Blackberry, I still don't regret getting the iPad. I would get the iPad 2 as soon as I have enough money saved for it.
Another reason not to choose another Tablet is the fact that you can find Tablet that used to sell for $400+, now available for about $150.00 or even less! Even the Xoom had to drop price because it's not selling as expected. If a product doesn't sell well enough it gets abandoned by its manufacturer. And certainly I do not want to be the proud owner of an abandoned product!
The iPad ecosystem has evolved rapidly, more than most of us ever expected. iOS has been gaining new functionality with a minimal performance hit.
On the other hand, the competitors keep throwing new versions of their tablets instead of supporting them. Just as with their phones, they release their newest product, sell it, provide maybe an update or two, and then focus on their next newest product and forget about the previous one.
That's why my HTC Touch Pro 2 phone's last and only firmware upgrade was an already outdated version of WM 6.5, then no more upgrades ever again.
And those are my reasons why I keep choosing the iPad over any other Tablet.