I switch to Shorai lithium batteries in both of our bikes a few years ago, and they've been AWESOME. They're expensive, for sure, but their performance is better than any lead acid battery I've ever owned.
I first bought one for my Shadow, because I was only getting about 2 years out of each battery. It turns out that I was killing the batteries by repeatedly draining them (I was using too small of a slow jet after doing some other mods, so starting was hard). I assumed that if the battery was dying, I needed a better battery. In the end, that wasn't the case - but the Shorai battery worked so well that I bought one for my wife's bike, too. The things I like about them are:
1. They crank hard. Both bikes fire up noticeably quicker with the lithium batteries than with the lead ones. The difference is probably a second or less, but after years of getting used to how long they take to ignite, you can definitely tell that it's faster with the new batteries.
2. The lights are brighter. I was kind of worried at first that I might burn something out, but after 4 seasons on one bike and 1 season on the other, no problems so far. Per my analog multimeter, these batteries put out about 13.7 volts.
3. They charge fast, and hold a charge for months. I haven't done quantified testing on the charge rates, but I find that after a difficult start that would normally run down my lead battery, these batteries bounce back after a short ride to town (10-15 minutes). After taking them off the storage shelf in the basement, where they've sat for the past 6 months with no battery tender, both of them registered 13.6 volts!
The batteries are also extremely light, and smaller than an equivalent lead battery. The weight difference is negligible in a 600 lb bike, but it's interesting to hold in your hand. The batteries come with a set of foam pads of various sizes to line the interior of your bike's battery box, too, in order to get a nice, tight fit.
Lithium batteries may not be for everyone due to the cost and reportedly bad cold weather performance (colder than you'd realistically be riding at), but I'm sold on them for our motorcycles.