Why do I need a computer to run my garden?
Short answer: you don't. Longer answer: computers are really good at responding to data. By using a computer to respond to your plant's needs, you can grow healtier plants with less work and easily share data.
Where can I find instructions to assemble a Garduino?
If you bought a PCB from me, the assembly instructions are here. If you want to work off of the original circuit sans PCB, check out the instructions here.
Why buy a kit vs. build?
The only advantage to a kit is you get a fancy PCB instead of running wires to a breadboard. But, you can go crazy customizing the breadboard version, ostensibly with less trouble...
What kind of soil should I use?
I've been experimenting with this, and don't have a definitive answer. The Square Foot Gardener mix is nice if you can get vermiculite and 6 kinds of compost at a reasonable price. I want to experiment more with hydroponics here...
How much plant growth out does one achieve per watt in?
Awesome question; you should build a Garduino and answer it:)
Is all this complexity really worth it?
As far as results in terms of increased plant growth, time will tell. For the learning this project's facilitated, definitely!
Solar / irrigation versions?
I built a solar irrigating version using a Voltaic solar panel, and I'm sure you can come up with cooler custom versions yourself.
What about sprouting?
Sprouting can be achieved with the normal setup. Also, check out this Instructable on grahamlawson's Properduino.
Can one use a similar circuit for animals/mycelium/children/etc.?
Sure; you should build it;)