<p>This free collection library comes from IT University of Copenhagen.</p>

<p><a href="http://www.itu.dk/research/c5/" rel="noreferrer">http://www.itu.dk/research/c5/</a></p>

<p>There is a video with one of the authors on Channel 9. I am trying to learn how to use these collections and I was wondering whether anyone has more experiences or what are your thoughts on this specific collection library for .NET. Do you like the way they are designed, do you like their performance and what were your major problems with them ?</p>

I've used it in the past and there are a couple of notes I must make:
The library is very good, very fast and very useful. It has lots of very nice data structures, some of which I did not know before starting to use this library.
It's Open-Source! This is a huge benefit.
Sometimes you don't have exactly what you want. As far as my experience showed, the library's authors decided to go with a very fault-intolerant attitude, throwing exceptions about everything. This caused me to add a few fault-tolerant methods.
All in all, a very nice library with some advanced data structures. Unfortunately, support for it is very lacking, as you can see from the fact that new releases (bugfixes, et al) range somewhere from 6 months to a year.
Note:
Starting with Mono 2.0, C5 is
bundled as a 3rd party API
, which I believe to be a wonderful show of faith in the product from the Mono team.