I used a Samsung D880, the only respectable dual sim phone I found, the rest where from unknown brands.
I found that I could not use packet data while making a voice call,also the carrier I used, had very high rates for gprs. The D880 is not a smartphone so I used it for tethering with my N810.
When I came to the US, I couldnt use the secondary phone of the D880 since it didnt support any of the US frequencies.
I found it very useful instead of carrying two phones around, it only cost me 200$ unlocked