I worked for European Centre from Sept 03 to May 04 based at their Jubail Office in Saudi. I spoke to some teachers who had worked in the Riyadh Office and they said that it was a lot tougher than Jubail. When I was initally offered the post I was told Riyadh as well but it was only when making the final arrangements that I was told Jubail so you may yet be based elsewhere. From what I can gather the Jubail side is the larger concern of the business.
It's basically down to what you expect and what you know already about the life and culture in SA. It's a very strange society and you end up wrapping yourself in a cocoon so life becomes very samey. Apparently life in Riyadh is a lot stricter than elsewhere hence making it a tougher gig than elsewhere but then Jubail wasn't exactly liberal central.
I went for 3 months initially, reached Ramadan and decided that it wasn't so bad so returned with the intention of doing a year. I lasted another 5 months and then had to take a break with the intention of coming back to complete my contract but once I got back into the 'real' world I realised that I just couldn't do it.
The school itself is alright with the local MD in Jubail being a bit of a pratt but in a fairly harmless way.
Let me know what it is that you want to know and I'll try and help you out as best I can.