Depends how much the money is worth to you. I like my Edge 800 (when it works) but don't use all the features. The mapping is rarely useful, but, when I ride out in some places I don't usually go, it can be handy. I never use the "virtual training partner" thing ... but the rest of the unit is still useful. If you won't need mapping in the unit, the 500 is cheaper and smaller. You can still create maps from your rides after the fact. For a lot of people, the data these things give you is useful, "but not during the ride."

I really like knowing how much climbing I've done at the end of a ride.