For my personal situation, the iPad was the perfect device for my needs, and is a good value compared to my other options for mobile internet.
I have an iMac for work (I'm a work at home mom), but before the iPad I was completely tied to my desktop. I do not like using laptops, and I don't typically need to do serious work on the go, so I've never bought one. I also couldn't justify the monthly cost of a smartphone. I use < 30 telephone minutes/month most months so I just have a cheap pre-paid phone. I did, however, need access to email, internet, and some files/calendars, etc. on the go. Before the iPad, there really wasn't anything that suited my needs at a price I could justify.
I now have internet wherever I go, and the $15/month plan is just right for me most months to supplement all the Wifi hotspots around. In months when I travel a bit, the $25 plan is more than enough. That represents a great value to me compared to either an expensive smartphone plan w/ a 2 year contract (which would limit me to a tiny screen) or buying a laptop+data plan (which would require lugging a laptop around everywhere and booting up the laptop every time I wanted to use it).
The iPad may indeed be a luxury for those who already have a smartphone and laptop/netbook, but for people who don't have either of those things (yes, we do exist, even at MacRumors!), it can be the perfect solution.