I'm on my third season with an LiFePO4 battery. Excellent performance (720 cranking amps!) in a 3 pound package, no need to have it on a maintainer through the driving season, though I do do a pre-season top up before it's first outing of the year. Hardest part of the install was making an adaptor plate to allow it to fit the battery tray.

The concerns about lithium batteries being a fire hazard are dated and more theoretical than actual. The current LiFePO4 type for automotive use are highly stable. I do have a battery disconnect switch which is off when the car is stored, but I've done that on all my hobby cars for the last 40 years. As with most parts, avoid no name brands that are suspiciously low priced.

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Steve
Late 2012 M3W