I agree - for the money, it's unbeatable and is a VERY handy little accessory when you want to practise or if your modding a guitar or just tuning it. I've tried it on Les Paul and Strat style guitars and it fits both very well, not even having problems with the Strat's recessed jack socket. Indeed, in one way it's better because the amp is held in place whereas with sockets on the edge (Les Paul etc), it can sometimes spin round or be prone to being knocked if you're playing sitting down or relaxing in, say, an armchair.
You are right about feedback with electro-acoustics, although personally I'm unclear about why anyone would use it with such instruments because, by definition, they are fully acoustic anyway. Maybe there is value in blending acoustic and electronic quality sound? Not sure.
A great little invention .... wonder why nobody thought of it before?