Has anyone else been over at Wattpad?
Honestly, I did not know that One Direction fanfic was such a big thing. Most site users are young, female and accessing the site from school according to Alexa (site stats).
Are there any other sites which you use to test publish or can you give me any advice about Wattpad - maybe it's not dominated by highschool/groupie fanfic after all?
It depends what you are trying to get out of the site.
If you are trying to build up a readership in order to get a publishing deal - nowhere is the "right" site for that. Statistically you'd do better putting your time into learning and improving and trying to acquire an agent.
If you are trying to get honest feedback - display sites (like Wattpad, Authonomy, etc) are not the place to do it. While there are undoubtedly people who know what they are talking about, there are far more people gaming the system and telling people who don't know how to punctuate dialogue correctly that they have written Teh Best Thing Evah(tm). Find writers communities and online critique groups - look for ones who have strong communities in your genre (because they will know about current trends and conventions) and who are actively submitting to agents.
If you are trying to use it as a platform to help promote a self-published work - possibly. It depends on what that work is. Have a look and see if you can find works in your genre, see if you can find the people you'd hope would buy your novel actively using the site or whether they hang out elsewhere - I know a hugely successful self-publisher who gets a lot of her audience via the Mumsnet community (which she was using anyway).