First: I wouldn't recommend it. Maybe I'm just old fashioned but I'd consider a real keyboard and a lack of distractions a necessity for serious writing.
That said, the notepad feature in 'my stuff' is handy to jot things down in on the go for copy and pasting later, but I've never used it without the intention to do some heavy editing afterwards.
But otherwise, if you're comfortable writing on your phone, any writing app will do. Even when writing on the computer it's not uncommon to use Notepad or Word and just copy and paste onto the site later. People have systems for organizing such as writing up one path at a time--maybe @Bucky or @Endmaster could tell you more. It's always going to be a little messy but you'll want to keep a backup of your work one way or another. Personally I prefer writing in Wordpad over the little pop up boxes, the only problem is when I'm trying to add to a story from separate computers or I need to get in there and do something with scripting.
