The Balmar Smartgauge is quite simply the most accurate state of charge meter available for lead acid batteries.
There’s nothing I can add that hasn’t already been said by Rod Collins in his incredibly thorough and detailed Balmar Smartgauge Battery Monitoring Unit article.
There’s also a ton of interesting and useful technical information at Smartgauge’s web site.
It was relatively simple to install. Most of the time went to cutting the hole in the panel.
I will say, though, that the fuse holders and crimp connectors supplied with the device are utter crap. Do yourself a solid and get some good ones. To learn which ones are the good ones, check out another top-quality Rod Collins article, Marine Wire Termination.