There's enough juice in the internal battery to charge an original Apple Watch up to six times (I have to admit I've not needed this - I've only needed three but that was no problem), and because it has a USB port you can also charge an additional device from it, but again, not a function I've used in anger. Clearly this wold reduce the number of Watch charges you could get from the battery.
There's room in the carrier for a couple of spare Watch bands which can be useful if you need to glam up for an evening do after a hard day in the office.
Best price in the UK is around £30 on the UK Amazon store. Recommended for weekend travellers.