Potential solution:
I started downloading 6.0.1, had the same problem. The progress bar never showed any progress, but I noticed in the activity of the system that the data was coming in. I waited until I saw more than 900 MB (download is only 850 or so).
I double-clicked the entry (next to the progress bar) and received a message stating that an error had occurred - try again later. I closed and reopened Xcode, and the HAD installation completed.
6.1.1 on Mavericks did the same, but 6.1.1 on Yosemite worked as expected.
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