I have been heavy (maybe _too_ heavy) into 15mm WWII for about 6 years. Here's what I've learned, and the opinions I have formed. I have posted versions of this on the Internet over the years when people ask about getting started in this period.

First, a few general observations. Each manufacturer has strengths and weaknesses - some are great at vehicles, but can't make people. Others are just the opposite. Scale, particularly for the vehicles, is pretty loose. Some lines are designed to be a bit bigger (Peter Pig, SDD), some lines are inconsistent with their scaling (Old Glory, some of the older Batlefront stuff). The infantry from most manufacturers seem to have settled at a size that makes them compatible, which is nice. Some, though, just don’t mix well with the others.

More 15mm Primer WWII Miniatures

Introduction
Quality Castings
Old Glory
Peter Pig
Battlefront
Essex and Feudal Castings
True North and SDD
Edwards and QRF
Gaming Miniatures and Outland Games