I love my MX510, it's an excellent mouse and it looks cool. Shiny red...
I originally had the MX500, which was superb, the button layout is the best. When I got the MX510, I thought the place that I bought it from just sent me a shell, because the MX510 weighs about one-quarter of what the MX500 does. The MX510 is so light, that it's unbelievable at first, you really need to get used to it.
I would buy the blue one from NewEgg.com, they have the cheapest price even with tax (I live in NJ and they charge me tax). They have free FedEx ground shipping for it. NewEgg.com does have the red one for $0.01 more than the blue one, but they charge $5 for shipping it.