I guess that depends on what you do in your life. As my lectures at university are a good 20 minute bike ride away and my laptop is too large and expensive for me to take it there and back every day, I use my iPad as a compromise for casual browsing and productivity while I'm at my lecture halls. The 7" form factor is more portable than the 10" form factor used by the regular iPad, so while a 10" tablet is good for browsing at home, it's kinda too big to practically take out with you unless you have a rucksack with you. A 7" tablet can, on the other hand, fit in a large pocket.
If you have other iOS devices, then the app crossover is nice. However, the resolution is a bit funny (I think it's 4:3 when the norm for tablets is 16:9 or 16:10) so be prepared to have black bars around the top and bottom if watching videos. So really, the best thing a iPad mini does is strike a nice compromise between the portability of the iPhone/iPod Touch and the screen real estate of the iPad. If you have one for free, then I'd hang on to it. You may find it comes in handy with apps or just for browsing on the go, and if it turns out you don't like it, you can get a pretty penny for it on Ebay.