Ion engines - can we really use them?

[RexKramer]

My experience has been they are not practical except for extremely light probes.

On the other hand, they are a great way to fill your tech tree for almost no cost. With a minor amount of planning, you can send a single ion probe to every body in the system, including a landing on many of them! A basic ion probe would consist of an engine, 3-4 xenon tanks, the tiny probe core, a battery, some solar panels, the 4 small science experiments, and an antenna.

I managed to get one to do passes of Eve, Gilly (landed), Duna, Ike (landed), Dres, Jool, Bop (Landed), Pol (Landed), Tylo, Vall, Laythe, Eeloo, Sun-Dive to low Kerbol pass, Moho (lucky intercept on the way up...), and back to Kerbin (another lucky intercept). Kicked myself for not putting a parachute on it for a safe return! Ended up with a spectacular crash on Kerbin, and miraculously everything was destroyed EXCEPT for the probe core! So I got science credit for the return of a craft from all the above-mentioned bodies.

Unless you are very, very patient, I recommend keeping the probe as light as possible- no kerbals or unnecessary mass. Burn times are not that bad on a very light probe. Anything bigger and I don't have the patience.