A sandel EHSI is a brilliant pice of kit. I flown an aircraft that had a sandel coupled to a 430 + STec autopilot. The auto slew feature is excelent when flying gps flight plans or approaches. No more twisting of dials for the new leg. The 70 deg arc view is also excellent when flying NDB approaches since the needle is bigger and deviations can be easily detected. It also has a map mode which is good for situational awareness. You can also display the same info as an RMI (if coupled to the ADF).
It also has some neat functions like sync and shift sync. If you want the autopilot to hold your present hdg, instead of rotating the hdg bug just press sync and the bug goes right to the top and press hdg on the a/p. And if you want to track direct to a VOR, pressing shift sync rotates the trackbar to the radial you're on the press nav on the. Little things to reduce the workload in the cockpit. You can also display stormscope info.
All this in a 3"X3" instrument. It isn't that cheap though aprox US$8k. But it's almost a waste driving some cheap 1960s junk HSI with £(tens of thousands) worth of kit.
Using the STec I've noticed the 1/2-1 bar deflection thingy too. I think it tracks a better if you intercept your track using hdg mode. But it does a good job overall, the Stec can fly a DME arc (GPS overlay) quite accurately.