So at the risk of perpetuating a stereotype, usually tablets are oriented at media consumption. Laptops are more targeted at productive work rather than passive consumption (or gaming). So how do you use technology? Do you consume media, or do you create content? I love having a tablet, but in my opinion the interface is not optimal for productivity-related tasks. However, if you get a tablet that has a keyboard (like the Asus Transformer, or something that can leverage a bluetooth keyboard) then you do get more flexibility. You have a nice media consumption device that can also double as a light-duty productivity device. Add in a nice remoting tool like Splashtop that gets you access to your home PC, and you've got a solution that I think is far better than a laptop for traveling.
The only real downside about tablets is that many of them use the 10" format. That's a great size for watching videos and browsing the web, but when using the device as an e-reader (aka, Kindle) the device gets a little heavy. I actually think that the 7" format would be ideal there.
As far as comparing a tablet to your phone goes, it really depends on how you use your phone. I always have my phone with me, so if I need to look something up quickly, read or respond to emails, check social networking feeds, etc, that's my quick go-to. However, my phone is my primary communications device rather than a media consumption device (for me). Watching movies on the phone is unbearable, as is reading an ebook. I hope I'm being clear here.
My third main device is my desktop PC. If I'm doing anything truly heavy-duty (development, serious gaming, running virtual environments, etc) then that's the device I use there. It's also where I do my most secure transactions (banking and shopping) because the likelihood of my losing my desktop PC is a lot lower than losing a tablet or phone.
So basically my hierarchy is:
Desktop PC: compute-intensive or security-sensitive workloads
Tablet w/keyboard (Transformer T700): watching movies, surfing the web, remote access to home while traveling, reading, maps, primarily leisure activities, etc.
Smartphone (Droid 4): responding to email, tracking my calendar, business functions, phone calls, and quick bits of lite media consumption (including social feeds), taking photos, etc.