We were there in 2004 during bike week with the fiver we had then and it was a blast. We had made our plans and committed to vacation time at work when we discovered that we would be there during bike week. The Black Hills are definitely something that you need to see at least once. We've been there twice and will go back again in a few years.
Yes there were LOTS of bikes and we were a little apprehensive at first but not once did we feel threatened in any way. The vast majority of people that we met were middle aged professionals like ourselves and were great folks. Many, many 40+' diesel pushers hauling enclosed trailers with two Harleys in them.(kind of wish that we could afford that many toys) While there were a few morons on the highway as there always is most were very competent and courteous drivers/riders.
Definitely don't go within about a hundred miles without a reservation though. We found that the "bikers" really added something to the experience. Lots of really cool bikes although I admit that after about 72 hours I did find that the sound of a Harley got a little old. As with most places just be careful where you go and what you do. Situational awareness is what it's all about. It is what you make it and it's something that you'll remember for a long time.