People keep telling me I should get an SWG, and I am tempted ("Get Thee Behind Me!").

I am curious: I've noticed that several claim you plug your pump into them and they act as the timer.
How does this work with a 220V pump, with a twist-lock plug?
Or does it?
Is it really necessary?
Do you have to have an electrician wire it in?
If your pump is on a timer, can't you just set the SWG to go on when the pump is on?
Do they run off 110v, 220v, or both?

I've also noticed that there are some non-plumbed versions, very cheap.
Has anyone tried them?
Do they work?
For how big a pool?

I've also noticed that some of the less expensive SWGs, ($999 and under) have cells that cost fully half as much to replace--like the Hayward costs 1K and the replacement cell costs $550! Is this a normal ratio, or does it get better with the pricier units? That is, do the cells last longer, or do they cost less to replace?

These are the details of SWGs that I am totally ignorant of.