Usability 
Wapedia currently has the best mobile version of Wikipedia. It's fast, always up and does an outstanding job of adapting Wikipedia content to everything from five year old wml-only handsets to an N95 or Opera Mini. It works just like Wikipedia on a PC, to deliver the entire Wikipedia online encyclopedia including all images, charts and tables in your mobile browser. Wapedia splits pages and resizes images so that everything fits into the physical screen dimensions and the memory limits of just about any phone
Wapedia has a very fine-grained algorithm for splitting pages and resizing images. It sets maximum image width to anywhere from 91px wide for the Nokia 6230 to 310 px wide for the iPhone or G1. The same is true of pagination. Wapedia dynamically adjusts page size to fit handset memory. The N95 gets Wikipedia's huge Tokyo article as 9 pages while the old wml-only Motorola i85 gets 60 much smaller pages.
Wapedia has advertising, although it's pretty minimalistic, just a single text link at the top of each page. I hardly notice the ads, but if the thought of advertising on Wikipedia content bothers you, try Wikipedia Mobile.