Seesmic is probably the best Twitter client for devices which are running this OS. It offers a very clear (high resolution) interface, combining light colors (where the tweets are received) with black colors (where are the options), and a red line in the upper part, which makes the application look better than any other Twitter client I have ever seen (on any other mobile OS).
The application doesn't just look excellent, it also moves excellent. Anyway, it has no reason not to do it since the device has 1GHz processor and 512 MB of RAM. There are plenty of options offered by the application, so it can be easily compared to a desktop client (TweetDeck for example).
The scrolling is not as smooth as it is on the iPhone, but the main advantage here is that if you move the finger across the screen fast, it scrolls down until the bottom of the list automatically. Plus, while you scroll down, the list of older tweets is automatically fetched.
The application is always running in the background, and I discovered this because I was woken up several times during the night every time I got a reply or a DM. Actually, the application is not making any kind of pushing because new tweets aren't loaded until you open the application, but it just checks periodically for any replies or DMs.
It has also a pretty annoying bug (or it's just a communication error with Twitter), which makes all the tweets which were sent up to 1 hour ago to appear as sent 1 hour ago. I mean, if somebody sent a tweet 15 minutes ago, it appears as sent 1 hour ago. I didn't observe any other bug into the application.
To get it, simply search "Seesmic" into the Market.