I don't think there is anything inherently wrong with individual using statements vs. using namespace, but using namespace saves some time when you are using a lot of different objects from the same namespace. If later you find out you have a conflict, you can easily switch to individual using statements...
The main point here is to put using something to make the code a little cleaner and easier to read.
Whether you choose individual usings vs. using namespace, it all comes down to: How much is your time worth? vs. How likely is it that a name conflict will occur in the file you're working on?
