I just read a question about octavers for electric violin here. So here are my impressions and decision.

Some years ago I tried various octavers and was not satisfied - the octaved sound had nothing to do with the original, it was synth- like and dead and technically the sound was fluttering: the units didn't get glissandi (slides).

I tried harmonizers: same result till I tried the really expensive ones: fine.

So I tried a third path: octave strings. Just marvellous!!! I tried Supersensitives, which only partly have magnetic properties (I have a magnetic REBO pickup installed on my baritone violin), then Thomastik Super Flexible (set 2714), which are perfect. The strings need a viola bow because of their thickness. Because of the thickness it took a while till I got accustomed, but now I'm absolutely happy. The deep sound is lively roaring - even acoustically. Electrically? Listen yourself:
Live:
http://www.youtube.com/watch?v=vrR3CsfD61s
Sketches (accompaniment MIDI):
http://www.myspace.com/dodecats