I've switched themes on my blog. Again. And, once more, I just yanked an off-the-shelf theme and slightly tweaked the CSS to make it do what I want.
They're both good, so why switch? I was messing around with Magazine Basic for a blog at the Teaching & Learning Centre, where we needed a more magazine or newspaper feel to it. And was struck by how much I liked the theme. I like that it's very clean, but polished. It only shows excerpts for the last few articles on the front page, and will show small versions of images if they're available for a post. I like that it's not purely river-of-posts - there's no "Older Posts" link on the front page. Once things trickle off the front, they're accessed via the category and tag pages. No humans use the "Older Posts" stuff, and googlebot has a full index of the site, so that design was redundant clutter anyway. I like that full posts aren't on the front page - that means more posts are visible at a glance, and most people won't even notice because they're coming from Google (directly to a post anyway) or RSS.
I'll give it a try for awhile. But so far, I'm really liking it.