Do a search on here. I think someone asked that question before. From what I have read, you save the weight but very expensive and they can fail if not properly taken care of, but that goes for any battery.
Most batteries will fail without the proper charging so Lithiums are no different. 50ah will not provide a much capacity for a trolling motor unless you run at low power levels all the time.
Yes, they are expensive, so whether you can justify them is a very personal choice. They do work, work very well and for a long time. Best application is as a starter battery for small outboards.