I currently do it. If you do it properly it's harder than it looks, but it's not rocket science. Besides standing in the heat, there's not a lot of physical work but it can be mentally draining if you're on a busy site.
I've answered your questions below based on what happens most often for me, but every answer is dependent on the job site you're on and the client your company is working for.
Is it standing up all day or is there breaks for meals, snacks and toilet breaks?
It's mostly standing up all day. Sitting is frowned upon and in most cases, even more dangerous. You're legally required to have 15 minutes of "other activities" every 2 hours to keep your mind active. This is normally a break, but the reality is that it doesn't always happen. If you're working for a good crew, they'll try to get you off the road while they're having their breaks but it's not always possible - you can't just walk away if half the road is missing.
Is a typical day based on 8 hour shifts or shorter times?
Most people are casual employees on an hourly rate. It depends on how long you're needed for. I usually get 7 - 8 hours, but you get the occasional short day as well as the odd 4 hour minimum. If the company has a couple of good contracts and you're a good controller, you can easily get over 35 hours per week.
Do you have to stay in one spot or can you wander around (while maintaining control) a bit to keep circulation flowing?
On most sites, you're in the same spot all day. I try to keep my legs moving to keep the circulation going.
The pay rates are mentioned in the ads, are they realistic or just a ploy to get suckers in and rip them off?
The pay rates can be up there if you're on a union site, but it's not easy to get on those and they tend to be even more boring. The states all have different awards but you can expect roughly $25 per hour, plus a daily travel allowance.