I'm a practicing lawyer, but am inclined now and then to read, and wonder about, the philosophy of law. Sometimes I read and wonder about it in the same sense I sometimes read and wonder about the "philosophy" of other things--with a combination of fascination and disbelief. The disbelief is the result not necessarily of scepticism regarding what has been written, but amazement that it has been written. Some of us have a colossal desire to explore, define and map out the limits of the otiose. Other times, I read it because I think it may have a use.
Since the philosophy of law addresses the law, it addresses something we must take account of a great deal of the time as we live out lives. Particularly in the United States, the law is pervasive, and administered by a variety of institutions and entities that are almost literally everywhere one goes. It is something that is not only useful to know, but it must be known because it governs our conduct and is in some cases accompanied by a sanction. In certain instances, it mischievously takes the position that regardless of whether we know it, we are charged with knowing it ("ignorance of the law is no excuse").
I find myself torn between those philosophers of law who are proponents of natural law and those who are positivists. By "natural law" I don't mean the view that good human laws are derivative of the laws of God imposed by divine command, and that those laws accordingly ought to be obeyed on that basis. Instead, I mean the view that good laws (laws which are beneficial to people) may be derived from the consideration of our nature and our place and conduct in the world of which we are a part. For purposes of this post, I will define legal "positivism" as the view that for purposes of determining what to do with the law it is most useful to analyse it as it exists and functions, not to theorize regarding its most perfect form and organization.
I'm rather leery of those who philosophize regarding the law yet have no significant experience in the law as a system. Those who participate in the system that is the law have more knowledge of it than those who do not. I suppose practicing scientists must feel the same way about the philosophy of science as a practicing lawyer (or at least this one) feels about the philosophy of law. There may be value of a sort to taking a detached view, but the philosopher of X often is too detached from X simply because he/she is ignorant of it as it actually exists. Most especially, ignorant of it as an operating system. This can lead them to hypostasize their own view of the law, and seek to impose it. An example of this tendency is Plato's view of the law, and of course the state, as the means by which we are marched in separate, rigidly organized platoons of differing purposes towards the good.
The law as a system has become so vast and all-encompassing it probably doesn't lend itself to the kind of categorization and defining beloved of philosophers. The Austinian view of law is far too narrow, for example, and has been justly criticized as being applicable solely to criminal law if to any kind of law. The law as an operating system should be considered just that, and this requires that it be considered in itself without consideration of what it ought to be; it is what it is, and we don't come to understanding something as it is by determining what it should be. This is the manner in which positivism can be useful. Establish how we may productively change the law requires that we consider what should be the case, and this is where sophisticated systems of natural law may be useful. But we can't understand how or to what extent the law should be changed or the extent to which it does or does not serve a desirable purpose unless we know it as it is.