Queenstown proper has little to recommend it.
The centre of the town is a strip of junky shops and restaurants meant to shake every last dollar out of tourists' pockets. 
However, the area around Queenstown is very nice, to the point that the town is very useful as a launching point for some interesting trips.
Queenstown is a base for people wanting to take several of New Zealand's Great Walks, the Routeburn Track, the Milford Track and the Kepler Track.  When I arrived in Queenstown (early February), the Routeburn was booked through April, and Milford was booked three weeks hence.  During my stay in Queenstown, I spoke with a British couple who told me that this week's Routeburn availabilities were booked when they inquired back in November.  This made me feel a little better about my policy of deliberately not planning this trip.   The circumstances which prevented my Routeburn walk were determined far in advance of my arrival in NZ, so not much could be done about it anyway. 
The change of vague plans in terms of the Great Walks led me to discover the value of the numerous Day Walks that DOC has organised .  Since I could not walk Routeburn or Milford, I walked the Ben Lomond day track, which was fantastic. 
I also took an overnight trip out to Milford Sound, which is a must-see for a trip through NZ.
On the trip to Milford Sound, we made a stop at the end point of the Routeburn Track.  This was a large metal shed with benches and tables.   There were perhaps 20-30 people sitting in the shed.  No-one was camping there, so this group must have been the day's output from the Walk.    Which means that this group of 20-30 proceeded through the track apace, all walking the same segments on the same days, staying in the same huts on the same nights.  While I'm sure that I missed some spectacular scenery along the way, and this is perhaps unfortunate, I also missed experiencing this scenery with a mass of 30 strangers.