Lots of good posts on the subject - go to the "aero-electric" list as
suggested by some to get multiple pro all-electric opinions.  On my ES I am
going to an electronic engine control, so it will have a redundant
electrical system, but still a vacuum gyro.  Why?  Mostly because I have
this (patented) pressure regulator that will operate vacuum gyros from a
turbocharged engine (mine is) with no vacuum pump.  So the plane will be
sort of a flying test bed/tax write-off.  Otherwise the all-electric scheme
would maybe be the way I would go.

However, I believe you can fly safely with the older, cheaper systems.
Here's what I did on my Cessna for just that purpose:  I have a vacuum
system with no backup and a standard electrical system (also no backup).
The backup for the vacuum system is my S-TEC autopilot, which I chose
specifically because it is all-electric, getting it's information from the
turn coordinator.  The on/off button for the A/P is on the yoke so I don't
have to reach for it.  I consider that to be  more effective backup than a
backup vacuum system (more expensive, too - but the vacuum system won't fly
the plane for me).  I consider the A/P to be almost a no-go item for an IFR
flight and I turn it on and check it on the ground as a pre-flight item.
The A/P is also the backup for the other single-failure-mode system; me.  I
show regular right-seat passengers (my wife) how to turn on the A/P to level
the wings.  For my ES I plan to install the Autotrak A/P system which has
internal electric gyros.  I do change the vacuum pump every 700 hours for
peace of mind, but I see no statistical proof that it is a good prevention.
I've had both pump and instrument failures, including turn coordinators,
over the years.

Gary Casey
ES project
C177RG