One thing that I have yet to see any player other than me do is use Shield Batteries and I have no idea why. The best defensive thing that you can do against ground attacks is to put 1-3 batteries at your entrance. While an try to stay near the batteries so you can recharge your units. For those who still think this isn't a good idea allow me to give you and example. Say you do what i see and you have 2 zealots at your entrance with a battery. Then 2 enemy zealots come in with an upgraded attack. The 4 zealots do there work and obviously the shields on your zealots hit 0 first, so you have them move back, recharge and get back in the action. Now your shields are at full while the enemy's are gone. Soon your shields are gone again, then you have them recharge once more, now your shields are full again, the enemy's are gone and their HP is probably half way down. You can pretty much figure who is going to win now. This also works good for larger numbers as well. I set up batteries at my entrance all the time and it works great.

CreepColony's Opinion: Bad thing about shield batteries are that they are pretty big, and take up some room.