I have just about every medieval (Western European) volume of the various Osprey books.

I love the images in most.  They are usually well-illustrated, and not just with the colour plates, but with photos, too.

However, having spent time on a historic arms and armour forum, I know the historic arms and armour community has mixed feelings about the Osprey books.  Some are definitely better than others, and there are times that a bit of speculation creeps into the colour plates.  I would caution using certain Osprey books as a sole source, but most are pretty good.

I actually have the Scandinavian armies volumes, and I thought I would add that certain Scandinavian members on the arms and armour forum where I used to hang out were less than happy with some of the information.  I guess the volumes tend to lump all the various Scandinavian countries together too much.  And, some of the plates didn't depict the arms and armour as exactly correct as some people would like.

Still, I like the images in those volumes well enough.  The good thing about many Osprey books is that they contain enough photos of medieval manuscript illuminations, brasses, effigies, and extant armour that you can compare these images to to the colour plates.