We had Stargazer on our I/O and I was very impressed and happy with the results. It holds ours within 2/10ths of the set speed. We are changing our prop to a 19 pitch and im sure it will get even more accurate. In pure butter it stayed dead on. We were running approx 1300lbs in ballast as well and it worked great.
I did not do the install myself in fears of voiding the electronics part of my 7 year warranty. It did look straightforward yes. The place that did it for me (Performance Watersports -- Highly Recommend) gave me an awesome price for install and everything so I went through them.
I did mine myself in my garage. It took all about 30 Min. Very straight forward. I really like the GPS feature. Easy to program. For water skiing, it gets a bit tricky. But for wake boarding and surfing, the programming is very easy.