Like others, I had older lights that did not produce much light, like the EL320, especially not enough spill light. I like the idea of more than one light, in case batteries fail in one. The Magicshine is good, I use it most of the time on the low setting and get at least 6 hours of run time before I recharge.
Like the OP, I also have some Fenix AA flashlights mounted, these are not as bright and have less run time, but are good backups, and are useful off the bike for camping, exploring. I have never owned a more expensive bike light, so I cannot compare except to say the Magicshine is rather low cost for a bright bike light. There are beam comparison photos out there on the web.

After 11 months, I have not had any issues yet with the MS battery, but all batteries eventually develop less capacity and then fail. In truth, I have had many capacity issues with the Nimh AA batteries I've used, so the charger I now use for them (like the OP) allows for the test of individual batteries, and refresh if needed. The MS battery pack does not allow for this kind of testing, but so far, no issues... The MS battery pack is four 18650 cells in series/parallel, so one bad cell mean half the capacity, which is still better than the AA battery lights where they are all wired in series, so one bad battery and the light gets very dim.