I find it very frustrating when the operating system I'm using – OS X, Linux, and Windows all do this – throw out the useful contents of the filesystem caches to fill them up with the contents of the very large files I frequently work with (sometimes even swapping out applications to get more space for memory cache!) – even though these files are almost always used just once.
This behavior, a natural result of the unintelligent MRU list design, tends to make these systems extremely sluggish during or after working with such files; typically it takes 30-60s to switch to another application afterwards, which is an incredibly bad trade off for the small probability of improvement in performance in future operations.
So it's a relief to see the nice and simple ARC cache mechanism OpenSolaris is using for ZFS, originally developed by some IBM researchers. We should all have this!