<t>If you are starting with something that has a .Length or .Count (such as ICollection, IList, List, etc) - then this will be the fastest option, since it doesn't need to go through the GetEnumerator()/MoveNext()/Dispose() sequence required by Any() to check for a non-empty IEnumerable sequence.<br/>
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For just IEnumerable, then Any() will generally be quicker, as it only has to look at one iteration. However, note that the LINQ-to-Objects implementation of Count() does check for ICollection (using .Count as an optimisation) - so if your underlying data-source is directly a list/collection, there won't be a huge difference. Don't ask me why it doesn't use the non-generic ICollection...<br/>
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Of course, if you have used LINQ to filter it etc (Where etc), you will have an iterator-block based sequence, and so this ICollection optimisation is useless.<br/>
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In general with IEnumerable : stick with Any() ;-p</t>
