A few comments, the 600 is not 2x times the weight but it sometime feels that way. When I first got the 500 I had hard time finding the subject, especially birds in flight, it comes with practice. Also I took more oof shots then sharp ones, long lens techinque is hugely important. check this article
http://www.naturephotographers.net/ejp0801-1.html Also you may want to protect you investment and cut down on some that glare and visiblity with a neoprene lens cover like LensCoat available at
http://www.warehouseexpress.com/
Cheers