I have a netbook (10"), a laptop (15" I think) and plenty of desktops, and an android phone (3.2" I think). I don't see a place for a tablet for me personally. I can't put it in my pocket so I can't carry it around everywhere (like my phone which has 3g and also wifi).
The netbook is easily portable, weighs a pound or two and is the size of a smallish hardback book. If I am going to be sitting in one place for an hour or two (meetings for work, killing time between meetings or appointments, auto shop waiting room, etc), I'll grab the netbook. Paid $200 retail a few years ago and still going strong with zero problems (software or hardware). Only complaint is it won't play HD video, so my HD video files that I watch on tv are not compatible. However if I am watching HD video, I want to watch it on a large display, not on a screen. Netbook is awesome because it is a laptop, just smaller yet still powerful enough to do all the productivity tasks I need (office suite stuff like excel/word/outlook/powerpoint, google docs, google calendar, internet surfing/research, etc). I have 3g data on the cell that can be a wifi hub to give my netbook internet where ever I go so I'm always connected (when wifi isn't available freely).
Downsides to my few year old netbook are the short battery life (1.5-2 hrs) and it can't do HD. Also, the power adapter is a little bulky given the small size of the netbook.
If I were looking today, I would probably get one of the $200-300-ish 11.6-13" netbooks/laptops that had a HD video card but still weigh around 2-3 lbs. Black Friday is coming up so I imagine you will see some. A big reason why I like the netbook or mini laptop over a phone or tablet is the keyboard. I can't stand to do more than a sentence or two on my phone. With a keyboard, even the slightly reduced keyboard on my netbook I can still hammer out long ER forum posts at 80 wpm.
The netbook has also been a good travel companion, making its way with us on cruises, to Argentina/Uruguay, and numerous other weekend/short trips for business and/or pleasure. If you fly much, it is small enough to stow in a bookbag, purse, or carry on bag, and not have a laptop case count against your 1 free carry on and 1 personal item limitation.
If I am working on a major task on the computer (which includes playing graphics intensive computer games), I'm using the 10x more powerful desktop computer with dual 20" monitors.
Obviously a personal choice based on what you intend to use the device for, and how mobile you want to be.