The Spinnaker Tower was originally called the Portsmouth Millennium Tower, but due to cost and time overruns it wasn't completed until 2005, making the original name somewhat irrelevant.
In fact, it still isn't completely finished as they couldn't get the external glass lift working and had to abandoned that idea.
The single internal lift therefore has to double up and can get unpleasantly crowded in busy times.
Is it worth the wait and the crush? Definitely. The views are magnificent, and the glass floor is fun.
For opening times, admission prices, etc. please see the official site listed below.