I have been using a 8800 for about a year now.
I recently purchased a 8520 and found it far to light and the keypad to small and hence after 2 days I sold it on.
I have just purchased a 8310 and although I prefer it to the 8520 its still not upto the build quality of the 8800.
To me the 8800 is perfect in size and weight. Not to heavy but you certainly know its in your pocket.
I recon the sim card will be coming out of the 8310 and going back in the 8800 by the end of the weekend