In an effort to be helpful, I'm going to give you my best assesment and opinion on waterbrushs, or water pens. Whatever you'd like to call them, it really doesn't matter.
As you can see, I've tried a few brands and I definately have my favorites.
If I'm being completely honest here, the Sakura Koi are my favorite. They don't leak, the brush bristles stay together well, they're just the right firmness and seem well made. They do screw on in the opposite direction that I'm used to screwing caps and lids on, but I'm not sure if that's a Japanese thing or if it's just a waterbrush thing.
The Derwent are hardly ever used because I have a hard time getting water to flow through the bristles without a struggle. The plastic is much harder , although the bristles seem well made.
The generic set of 5 has been a nice back up set for when I'm traveling and don't want to carry an extra bottle of water with me. They do have some leaking issues when you squeeze them. Water seems to seep from above the bristles, which can be a problem if you're trying to be precise. It also sucks when you drop water on your painting, creating a water spot. :( But as long as you're careful, you should be ok.
To use, you gently squeeze a tiny drop of water out onto you paint and brush around until you've reached the level of transparency you're looking for.
To clean in between colors, you simply squeeze out onto a paper towel or rag. I travel with a designated rag because I don't like to waste paper if I don't have to and the rag soaks up so much more water.
Here, I use my Sakura Koi Watercolors set, which is great for travel if you aren't looking to invest in tube paints or a separate palette.
It has a build in "dabbing sponge" that you can squeeze your brush onto for cleaning.
Some waterbrushes can fit snuggly into a pan palette center nook, making travel easy and convenient. Refilling is easy. You can go to a sink, or you can do what I do and use squeeze bottles of water to pour directly into the water reservoir. A small 2 oz bottle is enough water for most people to carry and paint a decent amount with.
I use waterbrush pens for almost all my sketchbook watercolor and gouache paintings, even when I'm at home. They're not just for travel. They are simply so handy that I see no reason to use a traditional brush for things pleasure painting.