Compaired to the spike and ferrules used just a few years ago Hang Fast hangers are superior on every level. I pay more for these but make that cost up on the money saved not having to replace gutters hit by a wild swing of the hammer. They hold up to 297 pounds each and ealily uninstall to perfect the slope of each gutter before leaving the jobsite.
We only use the latest and greatest sealant when it comes to making our outles, corners and end caps water tight. This is a 12:1 aluminum based seal that is self leveling. Once in place it flattens out into every crack and crevis leaving the gutter leak free while minimizing headloss on the gutters surface. Other sealants can do the job but most leave at least a quarter inch bead which acts as a water dam causing water to back up and sit stagnit around the corners and outlets.
For a quick estimate of new gutters