I had a T610 for 18 months and just upgraded to a K700 on O2 online contract.
No problems with phone calls, or branded menus etc, everything in fine working order. Voda did stick their logo all over my T610 which I could never get rid of, maybe depends on the provider?
As for the E800, I believe the E810 is coming out and is alot better than the 800, perhaps waiting a few weeks for? Never rated Siemens i'm afraid.