Yuck! :)
Seriously, it's an okay library that has two critical flaws (both with WebTextEdit - the base class that most, if not all, "text" controls inherit from):
1) Anything inheriting from WebTextEdit render 2 hidden elements and an input "text" element...three times the amount of HTML that I would normally expect...
2) I have yet to find a way to add attributes to the html elements output by the server controls. The HTML elements are created on the fly (they're not "child" controls), and nothing is done (what what I can see in the source) to ensure attributes are rendered.
If your pages contain a limited number of text-based controls, you should be okay. However, if your pages contain a lot (some of the pages I'm currently working on contain 100+ arranged in a grid) you could be looking at ~400K+ page sizes before viewstate if factored in.