I rented a Yamaha Silent Violin for a few months and ended up buying it. Different model number over here; I think the 100 in Europe = the 120 in North America. Essentially the same critter though.
I use it exclusively for practice, because it is a UN Charter of Rights violation for me to play any stringed instrument in public.
As a practice instrument it is dandy. We have a small house, and my regular fiddle, even muted, can be annoyingly loud to those with whom I share the space. The electric doesn't resonate, so plays very quietly. I play with or without headphones.
It is heavier than an acoustic vioin, with a fair bit of weight at the shoulder end, where the batteries and the electronic bits are. I have had a problem getting the shoulder rest aligned to my liking - it is a modified Kun rest.
The line-in is nice for practice, but if I am wearing headphones, have the fiddle plugged into the wall, and have a CD hooked up, I've got three different wires hanging off me that can get in the way of the bow. Not to mention create a tripping hazard in the event of a fire or empty glass.
There is a fair amount of background noise when it is on battery power, less so with AC. I don't know if this is due to dirty electronics or what.
A friend lent me a guiter multi-effects pedal - what a hoot! I use it mostly as a fun timewaster, but I'm sure you could create some nice sounds through it. Although I've heard that a bass processor might be a better choice for this.