The earlier Sapphires (830, 850 and 870) had some tension issues. I have an 850 and my issue was that I had trouble with FMQ. I blame my dealer for being unresponsive to me. The 835/850/875 seem to be better received. Many of the complaints were operator error. I think the computerized machines are less forgiving than mechanicals.
I don't know about the alphabet functions, but the machine is heavy - I wouldn't want to lug it very far. While I had trouble with FMQ, it sewed like a dream, and I loved the way the foot raises and lowers, and if you do any machine applique you'll find that a great help. It will do an accurate 1/4" seam, but you'll have to adjust the setting to make it happen. The Walking foot is expensive, but worth it - many times you can get the dealer to include one with your purchase.
Janet