I have an old Johnny Stewart electronic caller and it served me well. It's a cassette player, the down side to it is it's heavy and I can't find any good quality tapes for it anymore.
I purchased a Foxpro It's a lot lighter and works very well.
That being said I rely more on mouth blown calls.
Sometimes I use both. I'll start calling with a mouth blown call and use the Foxpro to coax them in closer and divert their attention away from my position.