They work great! When the battery started to fail on the XP, we kept that charger on it all the time when we weren't riding.
Just always be sure to use it at 2A (mine has 2A, 4A, and 6A settings).
Any higher setting and you risk frying something.
BUT you can use 4 or 6 if you remove the battery from the ski or disconnect all the cables from the battery.
Nowadays, I only use the tender once at the beginning of the season to recharge it from sitting idle all winter. Got new batteries in both my toys, so they hold a charge now.