I have to be honest, I wasn't all that thrilled with the first Mass Effect trailer I saw. In fact, I don't think any of the Mass Effect videos I saw actually made me think "Whoa! I must own this game!" I thought it sounded interest and I was eager to sink my teeth into a new RPG and, fortunately, Mass Effect delivered, in my opinion. I should probably be thanking Sharon, too, she's the one that brought it for me.
But, yeah, I don't believe there ever was a Mass Effect demo made available; maybe there will be for Mass Effect 2 but who knows. If any of your friends own a copy of Mass Effect then definitely ask if you can have a lend of it for a week or so. Alternatively, I bet you could buy it dirt cheap nowadays. I wouldn't be surprised to see Mass Effect going for around £20 on eBay. I'm sure you'll enjoy it!