When do I think iPad's will replace laptops?
Well if you mean the full functionality of a laptop - never - based on Apple's direction so far.
Apple is not moving the iPad towards laptop functionality, they're moving users away from laptop like usage patterns. Apple wants you to own an iPad AND a MacBook Air.
Now if the question were a more generic "when can tablets completely replace laptops" then I think the answer is today.
Before I say this I should mention what a fan of Apple products I am. I've had an iPad 1, 2, 4 (wife now owns) and a Mini 1 (which I still have). We both also use iPhones very happily.
However I just bought a device that for me, will likely be the end of my iPad usage for good. I bought the Asus T100 Transformer. Full Windows 8.1 in a convertible tablet/laptop for $379. I should mention I'm a PC geek, engineer, and make a living in PC IT s/w packaging/distribution/etc.
For what I do on a tablet, this one is easily the best I've ever had. I use my tablets primarily as portable web surfing boxes, about 80% I'd say. The rest is similar reading type activities - Facebook, Flipboard, Kindle, Reddit (reader apps), etc. Email/Calendar, some light video (Netflix/Plex), and a few light games pretty much rounds out my tablet usage. All of this is available in the finger friendly/tablet apps for the Windows Metro UI.
The Asus is the best portable web browsing tablet I've ever had simply because it is a REAL PC, with a full PC level browser (Chrome) running on it. All my extensions (no ads - yay AdBlock), no tab reloading, not a single browser crash in the three weeks I've owned it, downloading, uploading, flash, Java, etc. It's all there and it all just works.
I use my laptops for everything above (as needed) plus the usual - device maintenance (iOS, GPS, Android flashing, etc), printing, remote connection to work servers, iTunes (lol), DVD burning, USB drive tasks, torrents (shhhh), etc.
It's fast as my y/o Core i3 Lenovo (which I gave to the wife), lasts over 10 hours on battery, cost less than an rMini (64GB) and for the first time in my tablet using life I don't ever think that "I'll work on that when I get back to my PC" - cause I'm already ON my PC.
If only Apple would create an iPad that you could drop into a keyboard dock, and click an icon to launch a full OSX environment - with shared data on the iPad side. Man they'd sell a million of them...oh wait, they already sell more than a million of them without that. Oh nevermind...lol
I think these new Windows slates, that can do both intensive Windows x86 apps and lighter tablet consumption apps are going to become very popular and give Apple's iPads a good run for the consumer dollar.
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As I have been playing with my Dell Venue, having access to the file system, having to do driver and patch updates, running EXE files in my download folder, choosing which folder to install, etc., all that is like a warm blanket for me. I missed it.
This line made me laugh, and I can relate as an older Windows IT guy. When I got my T100 I felt the same way. Finally a tablet with a proper command prompt and security patches every Tuesday!!!