I was in Brownsville, Texas for two weeks in January 2010 for work. I found it to be an unpleasant city with little to offer visitors or residents.
A loto of the Brownsville population doesn't speak English due to its proximity to the Mexican border, and while it may still be America, it isn't the cleanest of cities either. The bars and clubs in Brownsville are riddled with prostitution and the drug trade and are dangerous to the newcomer.
Matamoros, Mexico is a main attraction around Brownsville, but the border region of Mexico is a very dangerous place for American tourists at this time and it isn't advised to travel across the border. However, there are some great Mexican restaurants in Brownsville, and most of the major retail stores like Wal-Mart and Best Buy are available. Most of the people in Brownsville seemed friendly, but the drug trade and poverty are prevalent and this can be a dangerous place for tourists and visitors. I wouldn't advise choosing this as a vacation destination.