He's a reasonable writer, but his prose quality is not fantastic. This is probably because of his insane output (at one point he was putting one 500+ page book out in each of three different series annually for 4-5 years in a row, an output that makes even Sanderson look like a snail).

He's got some great ideas, but, like most alt-history authors, his story becomes less interesting the more it veers from reality. In the WORLDWAR series (aliens invade Earth in 1942!) the opening four-book arc is entertaining and worth reading, seeing how things changed from reality (the difficulties the Germans and Russians have working together against the aliens, and how easily they turn on one another). By the later books, set decades after the point of divergence, things have become far less accomplished.