Iíve been running with a Lithium battery in my Softail and my wifeís Sportster for a few years. I doubt I would go that route again. Yeah, they are very lightweight. But they do require a special charger that can handle a Lithium battery. The bikes arenít modified and handle them well. My Softail doesnít mind the Lithium as much as the Sportster does. They drain down much quicker in off weeks of no use. You simply have to keep a Lithium tender on the bike if you donít plan on riding for a few weeks. The Softail will kill the battery in two to three months while the Sportster will do it in 6 weeks. The security system uses some power and the lithiums just canít take that long term draw like a lead-acid can. Other than the storage issue, the bikes donít care. The motors spin over easy, plenty of cranking speed. Letís be honest, I canít feel any weight difference on the bikes either. They are also expensive. More so than a lead-acid or gell-cell.
So for me, I probably wonít do them again. The benefits (weight and charge/recovery time) donít overcome the drawbacks.