I started with a silk DreamSack but got lost at some point and I replace it with a different brand sleep sack, but still made of silk.
I bring it whenever I am going to say somewhere that does not provide linens, such as a hostel or CouchSurfing. To stay in a hotel, bed and breakfast, or other not-quite-roughing it experience I do not bring it.
It does not take much room to pack as it is the size of a pair of balled socks.
I also use it when camping as a bag liner for purposes of keeping my bag clean.
It is a useful bit of kit, but if your travel style isn't going to change and you haven't needed one yet, I'd say the money could be better spent elsewhere.