Back to the initial question: replacing that aging laptop with the latest and greatest tablet, it's really a very personal matter and a careful measure of individual needs should be considered.
As a designer and web developer, I have a very wide range of devices for different purposes. My MacBook Air is my constant companion and central to my daily workflow, but occasionally my custom built and multi-screen Windows PC helps me handle the heavy lifting.
My iPad is a creative device (not just for consumption despite what anyone says). This article is being written in Evernote on my iPad, and I also use my iPad heavily for music production - sketching out song ideas has never been more fluid thanks to a plethora of incredible synths, sequencers, and unique instruments available on the App Store. I also use the iPad for illustration, and for creating quick web or application mock ups and the interface is very capable at allowing me to focus creatively.
My content consumption device of choice is my Nexus 7. I read, I watch movies, I use apps like Flipboard and Pulse to follow Twitter, and my favorite blogs. That said the Nexus also acts as my portal to all my Google productivity apps. Gmail, my calendars, my Google Drive, Android had a lot to offer on the productivity front if you're already plugged in to the Google ecosystem.
I know what you're thinking, it all seems like a lot. Keep in mind though, I don't really carry a smartphone these days. I also don't own a television, and these 4 devices are also shared in my household (my partner is working on our MacBook Air right now).
The point is, finding a mix of devices which works best for you is highly personal. Is a tablet alone a good replacement for a laptop? The simple answer is no. No you can't simply plug in a USB key, no you can't store all of your files on it, no it doesn't have a physical keyboard or windows. The reason it's more complicated than that is that a tablet offers a different way of working around those needs. For example, Why plug in a USB key when you have cloud services like Dropbox? A tablet fulfills other needs that a laptop could never, in new ways that just weren't considered years ago when an OS was nothing more than a file manager and a bunch of windows.
This holiday season you might be considering buying someone you care about a tablet, or maybe a new laptop. It's a beautiful gesture, but here's the thing; you're going to want to be careful to consider a few things about how that device will be used.
Syncing music to their phone, watching YouTube and using Facebook, these are some of the things that are most commonly accomplished with a computer, and you know, it's kind of a waste. A tablet, or a smartphone with the right mix of apps can handle all of that with so little effort, a MacBook Pro, even the cheapest model, is really just overkill.
School projects, note taking, essays and reports; maybe a tablet works (in fact, I'm almost certain if I were back in school I would have preferred it to the beige box I worked with) especially when paired with a great keyboard.
Now what about real work? Productivity usually means a bulky work laptop and Microsoft Office right? Well despite what Microsoft might have you believe, things are changing. I haven't used MS Office in years thanks to Google Drive and it's suite of highly compatible apps, and I won't even bother with the lengthy list of cloud services I use to get work done everyday, all with their own beautiful, easy to use apps for smartphones and tablets.
The problem here is that these are simply arguments for or against one technology or another, when again, it all comes down to personal use. I wish it were as simple as a questionaire you could fill out that told you what to buy, but it isn't.
At the end of the day the only simple truth about technology is it exists to help us solve problems. While a hammer does better at driving a nail then a pocket knife, desiding which one is better in general is impossible, and its up to the person holding the hammer or the pocket knife to decide what lets them best solve the problems they alone are faced with.
So consider the following: if that special person on your list must simply have a new tablet, laptop, PC or smartphone, don't make that decision for them. Be careful to consider why they want what they want, and if possible don't just surprise them, help them pick exactly the tool they need with some very careful consideration. Research the apps available, look at specific needs and how specific devices cater to them. Realize that in this age of ubiquitous computing it is a very personal choice, and one to be made with more careful consideration then one might buy your average appliance.
A quick anecdote. A few years ago, my family all pitched in to buy my 80 year old grandfather a laptop. They setup a Facebook account for him, they setup an email address. Three years later, what problem does my grandfather use the laptop to solve? Playing solitaire without shuffling the cards.