I have to agree with Levelord. The ion engines are not under-powered, it's just that the burn times are very long. I used them once for a interplanetary probe, but after 45 minutes of burning to circularize around a planet i had enough. Now i only use them for probes/satellites in Kerbin and Mun orbits. The best solution would be to enable time warp when using ion engines, but i'm not sure if this is possible without drawbacks for the calculations done in KSP.