Give each category a decent description, and use HeadSpace to add that description to the meta description, by adding %%category_description%% in the Description field. After that, write a description for each post or page that you actually want to rank with. The descriptions has one very important function: enticing people to click, so make sure it states what's in the page they're clicking towards, and that it gets their attention.
Automated descriptions
In my opinion, auto generating descriptions is a load of bull, most plugins pick the first sentence, which might be an introductory sentence which has hardly anything to do with the subject, or another sentence with a keyword in it, which might be completely wrong to pick as description. Thus, the only well written description is a hand written one, and if you're thinking of auto generating the meta description, you might as well not do anything and let the search engine control the snippet... If you don't use the meta description, the search engine will find the keyword searched for in your document, and automatically pick a string around that, which gives you a bolded word or two in the results page.
Auto generating a snippet is a "shortcut", and there are no real shortcuts in (WordPress) SEO (none that work anyway).
Automated descriptions
In my opinion, auto generating descriptions is a load of bull, most plugins pick the first sentence, which might be an introductory sentence which has hardly anything to do with the subject, or another sentence with a keyword in it, which might be completely wrong to pick as description. Thus, the only well written description is a hand written one, and if you're thinking of auto generating the meta description, you might as well not do anything and let the search engine control the snippet... If you don't use the meta description, the search engine will find the keyword searched for in your document, and automatically pick a string around that, which gives you a bolded word or two in the results page.
Auto generating a snippet is a "shortcut", and there are no real shortcuts in (WordPress) SEO (none that work anyway).