As it happens, I am "in the market" for such a product ever since the demise of Google Notebook. Having given it a quick spin, I may consider using your product. The only immediate irritation is that it isn't obvious how to move an annotation from one folder to another in the sidebar; it looks like drag and drop should work but it doesn't. The documentation (ie the FAQ) didn't cover this either. Oh wait. I managed it somehow but bugger if I know how - I can't undo it. Ah. I see. You have to grab exactly the right part of the annotation - the title (Not the URL, Not the Favicon). The design doesn't lead you to this because all three are in the same grey box, making it look like one drag-n-drop unit.
This is my opinion only, but I think the price point for your premium service is too high. There are two reasons I pay for premium service: one, I really really want the extra feature; two, I have a really good feeling about the service and want to keep it alive. Ten bucks a month hits neither of those for me. In comparison, I pay toodledo.com $15 a year for a really important-to-me feature and awesome customer service. Looking at webnotes I would get by with the free features, but if I liked it, I would pay you $15/year even though I am not interested in PDFs. With your price point at $10/month, I'm just not going to. I mostly use this kind of service to collect and organise recipes - that is nowhere as big a deal to me as half my Netflix subscription, which is what $10/month comes to.
The .net address in unfortunate. People do type "webnotes" in their URL bar then get confused when they don't go where they intended. There is no obvious reason why this is a .net kind of site.
I get some overwrite issues on Firefox/Linux in the sidebar. Not a biggie.
Hope this was useful to you.