I used to have a notebook with a built in wireless card and thought it was the best thing ever. Until that wireless card gave out on me twice in a year. The second issue is there was no way to share the internet without a huge rigmarole in network sharing. So I have a Mifi and a Wifi Only iPad. Here are some reasons why
1 - 3G on ALL my devices, notebook, iPhone, iPad
2 - I have the Verizon iPhone and I can call and surf the web at the same time.
3 - Local area network always available so I can use programs like Snatch to control my PC with my iPhone, and VNC to and from various devices.
4 - The Mifi has it's own battery (and I bought a Sedio extended battery so it lasts 10 hours too) - corollary - All my devices last longer.
5 - Upgradeable - the 4G Mifi's are now out - my Mifi contract is up in August - so in August I will have a 4G iPad and iPhone and notebook
6 - Since it is wifi, there is no connect if the network is remembered. So as long as the Mifi is on - the iPad just autoconnects when it needs network.
Secondly - everyone complaining about carrying a Mifi - how do you carry your iPad? I don't have pockets big enough for just an iPad - so I have a small bag - in which my Mifi fits. The Mifi does fit in pockets too if I just want to hand carry the iPad. It's just another device like a phone - if I have space to carry an iPad I have space to carry the Mifi.