When I first started painting with watercolours (about 7 years ago), I purchased the Koi Water Brushes with the small water wells.
The larger well on this brush allow for longer sessions in between water replenishment which I really love. The bristles are quite fine considering what they're composed of (very different than a typical watercolour brush, more plastic-y) and come to a decent point. The brush cap is genius - no more squished bristles - just don't lose it! Because the cap is clear, if you drop it while painting outside it may be hard to locate on the ground- a tip is to paint half of the cap a bright colour - that way if you drop it you can find it more easily. Also (just because I didn't know this), the black cap little **** cap thing it comes with is supposedly meant to be a water stopper, so you can take the brush apart and travel with the water loaded in the barrel. If you are working with a tiny kit, this is ideal because when fully built the brush can be about the length of a standard short-handle brush. So don't throw that away if this is something you may find useful!
I do wish the water flow was a bit better (ideally, if it was more similar to the Pentel Colour Brush for ink flow) then it would be great. When I squeeze on the barrel I find that I either get wayyy to much water or not enough - I just haven't found the sweet spot yet.
Overall, if you are looking for a water-brush to take with you while painting outside I think it's a good option, both for function and for variation in sizing.