OP I have been visiting Saudi for business for 25 years mostly Jeddah and Riyadh. I know a number of people well who have worked there, if you send me an email (in profile) I will put you in touch.
Firstly 3-5 years. Most people sign up for 2 year contracts, to do so for longer is very unusual. Be very careful about signing up for 5 years and thinking you can get out of the contract sooner if you don't like it.
You live there inside a European compound so it's a bit more relaxed but it can and will feel like a prison at some stage.
It is normal for your employer to retain your passport, this means you cannot travel without their permission.
In the past it has been very unsafe, one friend of mine was in his compound when it was attacked by a suicide bomber in a truck. I have also met the BBC journalist John Gardiner, you will see him on TV, he described how was paralyzed from the waist down after being shot waiting at a cash machine (ATM), he was accompanied by Saudi official security but that made no difference. I have made a few trips where we have been accompanied by security and searched for weapons in every building we visited. This personally makes me feel less safe.
In my experience people go to Saudi for a short stint and for the money with the potential to get valuable experience in order to boost their careers.
The rich is Saudi are very rich but there is huge poverty in the country too. We are starting to hear stories linking ISIS with Saudi and they are on the border with Iraq.
EDIT: By the way another mate of mine who works on the underground (MRT) in Singapore was approached about a big Riyadh job. I imagine this might be the same project