I use and have been rather enjoying my time with
Serendipity.
Before I comment further, let me say that I haven't experimented with many blogging platforms, so I'm not in much of a position to give comparative thoughts.
Initial setup was less easy than I might have liked--although some of that might be my own fault. It's perhaps worth noting here that I'm somewhat inexperienced in website setup, especially the back-end elements thereof.
As to theming, Serendipity comes with a variety of themes, with variable degrees of flexibility. (Most are fairly inflexible, I seem to recall.) With some time and willingness to delve into CSS, at least one of the themes can be reworked a fair bit, albeit within constraints (this is how I themed my own site, as I recall). I think that the functionality to create one's own theme exists, but haven't looked too deeply into that--I think that it may be a bit more involved.
Writing and editing articles is pretty straightforward.
There are a variety of plugins available that extend Serendipity's base functionality, both sidebar-related (such as adding a Twitter timeline) and back-end (such as adding a means of creating reusable page-chunks).
I have encountered a few issues. Two that come to mind are these:
- The media manager doesn't seem to like animated GIFs (I've ended up showing them via embedded tweets instead)
- I haven't managed to get the Twitter plugin to add hash-tags when announcing a new blog-post
Overall, I rather like Serendipity. ^_^