My HTC Desire HD is by far the most expensive phone I have ever purchased and is by far the best smartphone I have ever owned. It has relegated my laptop to a typewriter, something I use to upload large files to the web and a mass storage device. Still it will not replace my laptop. A tablet would not replace my laptop either. And that is why I am not big on tablets. It still won't replace my laptop, and it won't replace my smartphone. Two devices is enough for me (well actually I have three devices the third being a second laptop, a nearly four year old HP Compaq 6510b the is not worth selling and refuses to die on me).
I got my wife a Samsung Galaxy Tab last January. Earlier this month her nearly two year old Nokia E75 died. Instead of getting another smartphone to replace the E75, she opted for a dual SIM feature phone. Call and text from the feature phone, while the tablet takes care of email, mobile web browsing, instant messaging and acts as an organizer. The tablet keeps her laptop at home, but she still needs a computer.
What I am saying is that, if you get a tablet, you still need another PC. If you have a desktop, never decided to get a laptop, I think a Tablet makes a nice add-on. If on the other hand you have a laptop or even a humble netbook, your tablet probably wont replace either device. Instead of buying a tablet and a cheap laptop, a higher priced laptop might be more advisable.
I also think 10-inch tablets are the wrong way to go. A 10-inch tablet is a device you use on a table, and not while walking around or standing up. So why not a laptop instead? Apple iPad sales figures say I am wrong. But I am not sure how much of that is because it is an Apple, or because it is the cheapest Apple "persoanl computer" alternative. If Apple ever offered a US$499 iOS or MacOSX netbook, I think that would sell really well too. Maybe even better.