Since it was added to ArcGIS I use the IN_MEMORY workspace regularly in my day to day work. It's great for making temporary datasets which you want to have a quick look at and not worry about deleting later. A good example is the Spatial Join tool which always creates a new output dataset.
But I find it to be a very unstable environment; several times in the past ArcGIS has crashed spectacularly corrupting my Geodatabase. I can replicate the problem and it's always to do with the in_memory datasets.
Whilst I appreciate it cannot support advance features (e.g. geometric networks) I often want to join the in_memory table to a featureclass. When you try to join data ArcGIS always suggests adding an index which significantly improves performance but it appears an in_memory table cannot support an index so any joins and subsequent field calculations are painfully slow. It would be good to see future versions of ArcGIS supporting attribute indexing on In_Memory datasets.