Totally depends what you are looking for. 500 is great if you don't want maps. 605 is good if you want maps, but don't need hr, cadence, or the ability to share routes in the parking lot. 705 gives you every feature available, including maps, hr, cadence, power (if you have a specific crankset or hub), and route beaming. 800 looks good, but is overpriced given that it lacks the ability to beam routes in the parkIng lot (which is incredibly useful). 705 is the most common in our club, especially now that the price dropped.

I have to go with what Mark said. I like the 705 and ebay is running some good deals right now. There are some that are referbs but they still have a one year warranty. Most of them include the heart rate sensor and cadence sensor for 299 which is not a bad price.

I to have a 705 and really like it for all the above mentioned reasons. Downloading in the parking lot is one of the good options. If you buy one on ebay make sure that you get the maps chip with the 705.