The sound of the Behringer Truth B2030A studio monitors is about average at best. They pick up frequencies starting at 50 Hz, so they don't have the greatest low end response, but still pack some punch down there. They have a pretty good mid and high end response as well, so in general they are pretty well rounded and do give you a good dynamic response as well. While I wouldn't say that these are perfect for mixing, you could do it if you knew them well enough and didn't have anything else to use, I'd just highly recommend checking the mixes on better sounding monitors when possible. I'd say that they are best suited for other types of uses like for monitoring and editing, but I guess you could mix with them if you wanted to...
The price of the Behringer Truth B2030A definitely makes up for all of it's short comings, but even so I'd probably recommend shopping around before making a choice. While these monitors will most likely be fine for most smaller home studio owners looking for a budget pair of monitors, I wouldn't recommend them to anyone looking to put together a legitimate studio where you could bring in clients or anything like that. These days we have a ton of different monitors out there to choose from in this price range, so while these will seem pretty enticing, I'd do yourself a favor a do your research to insure that you're getting the best possible monitors for your money.