I'm of two minds about this. I don't mind the symbolism behind an engagement/wedding ring, but I don't like the "I have to have a huge rock or I won't marry you" mentality that some women have. I especially hate the "you should spend two month's salary on an engagement ring" dreck that jewelry stores try to shaft you on.
My wife's engagement ring is not the typical diamond solitaire. It is a custom-made 18k white gold band with a half-carat round-cut sapphire flanked by two 1/8th carat pear-shaped diamonds. It was not horrifically expensive and she loves it far more than she would have one of the cookie-cutter diamond solitaire rings that the jewelry stores try to get you to buy. Our wedding rings are quite simple, and both of them together cost less than her engagement ring.