I find his writing to be amazingly clear. While he made some mistakes in his work, on the whole his "critical thinking" is among the most persuasive I've come across.

E.g. [link|http://www.popular-science.net/books/russell/chapter8.html|Chapter 8] of his book, The Problems of Philosophy talks a little about Kant and his contributions. The whole book is there. (Note that you'll have to understand terms like [link|http://dictionary.reference.com/search?q=a%20priori|a priori], so have [link|http://dictionary.reference.com|http://dictionary.reference.com] handy.)

Cheers,
Scott.