It's not (reliable).
Sad truth is that there is not a single reliable source on the conflict, if you want to kind of know what's going on there, you need to follow a bunch of sources and go through a lot of propaganda to draw the median lines, if possible, over what happened.
EDIT 1: explaining the reasoning of the first claim
According to the Wikipedia article, that organization is composed of 1 (one) guy, a former anti-regime dissident living in Coventry:
The organisation is run by Rami Abdulrahman (sometimes referred to as Rami Abdul Rahman) from his home in Coventry. He is a Syrian Sunni Muslim who owns a clothes shop. Born Osama Suleiman, he adopted a pseudonym during his years of activism in Syria, and has used it publicly ever since.
This should be a warning signal. As for his track record and impartiality:
SOHR has been accused of selective reporting, with AsiaNews saying that they covered only violent acts of the government forces against the opposition for the first two years of its existence. SOHR has also been accused of reporting militant anti-government fighters among dead civilians, and has been described as being "pro-opposition" or anti-Assad. It has been criticised for refusing to share its data or methodology.
Or in the words of a Reddit user which matches my experience:
It's decent. It's a pro-rebel page so it's fairly accurate in reporting on opposition gains and news, but even then errors appear often and sometimes the page makes claims that end up being totally false. There are worse sources out there (SyrPer or Malcolmite come to mind), but there are also better sources. I would avoid taking SOHR's word for granted if you see them make a claim or report a victory/defeat.
Also, it's just one dude living in England. Idk how he gets his sources, but it's not a group - it's a single person.
Thing is, I could find you instances where he clearly reported something wrong, but you could say that for any source in this war. Reporting, for example, dead militants among dead civilians, inflating the military casualties of one side and deflating the civilian casualties from the other and vice versa - they are all common and honest mistakes in any conflict that anybody meaning well and with limited resources could make.
Patterns only emerge after following a source for a while. You notice one side never loses villages, just keeps conquering them over and over again :P or that children die of bombs only in their controlled territory or...
It's not just SOHR, try following any source for a while and you will notice most of their on-the-ground data or twitter sources, and possibly their allegiances seem to fall into wide categories, pro-government, pro-kurd, pro-isis, pro-rebel.
That's why my suggestion is to follow the situation closely, with a bunch of conflicting sources and see for yourself. In this conflict almost nobody will spoon-feed you real information for which everybody will agree that it's The Truth.
Open question, of course is, if my suggestion to you, a mere SE user is to follow a bunch of sources, and this one seems to be The Source for not only a lot of journalists, but the US government and UN bodies, how can this be?
Answer is probably politics :P journalists will typically (with some rare and honorable exceptions) pick up a partisan source and then retweet the hell out of it, SOHR for the rebellion, Yusha Yuseef for the regime, Syrian Rebellion Obs for Rojava Kurds etc.