Love HandCent, I've been touting it around here too.
When you're in a thread with a person, the settings button in the top-right will allow you to make custom settings based solely on this person. These include: person-specific notification sounds, custom vibration patterns, and even different led colors. This is huge since if I've got a specific vibrate pattern for certain people, so if one of the girls who are important to me texts during a movie, I can tell it's her.
In the main settings you can also choose a different LED color for all texts. So my texts flash blue (whereas other notifications are green, so I know it's a text). But any person-specific rules I made take precedence. For example, the same girl I wrote about above, she shows up as a magenta/pink color in the LED so I know it's her.
You can also change all of the colors and the style of the boxes. I have a lot of black backgrounds in mine, which makes it easier on the eyes in the dark and will help with battery life once I get a Samsung Epic.