I have read three of his writings, and have found them all to be quite good. One, concerning the great Shism, was very helpful in helping a Catholic relative of mine who is looking at Orthodoxy but had never looked deeply at the history.
I also found what he wrote about Judaism and the Byzantine empire to be refreshingly honest and objective compared to a lot of what passes for scholarship in world orthodoxy and heterodoxy when it comes to that subject.
I have downloaded the first part of his modern history of the Orthodox Church but I have yet to get around to reading it.
He seems sincere, and sticks to the facts, but I do not share all of his conclusions in regards to the Church and where grace is, or is not, and this may point to some bias in what he writes, but with discernment and care, I think any one can benefit from his efforts. I would say he has an attitude of super-correctness, but I wouldn't hold it against him, I have suffered from it also, and I hear he is very Christian in his manner and the way he relates to people. He gives all his work away for free and that is commendable in itself.