The Acoustimax is a great addition to an acoustic guitar. In my case I usually play a Gibson J-200, but with a passive under-saddle pickup in it. The sound is a bit thin and doesn't translate the rich sound of the guitar well. The Acoustimax fills in the sound a lot and allows for major EQ to get the sound dialed in. It also just makes the sound have much more depth or presence.
As far as the GT-X, My voice needs a little help. The doubling feature really makes a huge difference and I feel much more confident in what I'm projecting. I also love harmonies, but since I'm usually a solo act, this allows me to have some fun. The tones are really pretty good, but I found that if I'm not on pitch, the harmonies will sound off. That's actually a good thing - it's forcing me to be a better singer.
I also have to mention the little micro-PolyTune tuner. I got it because of space limitations, but I really like that little sucker!