The books you'll want to buy from Osprey are the very specific books, the books that detail a single battle, type of soldier or army. They are very detailed and good on sources. The ones that try to explain things on a "larger" scale are of a lesser quality however.

IMO.

i must have by now around 100 books, mostly MaA and Elite and few warrior and Campaign series. there are only 5 books or so i got that i was REALLY dissapointed with.

Age of charlemagne is good, Armies of islam too, but theres now a better oen around, armies of the calipahtes which covers the same period but is more specific and up to date, maybe too specific.

I find the Osprey series more than decent, especially the MaA and Elites series, but I am fairly dissapointed with the campaign series. So far I've read two of them (lended from a friend to write some articles) by the same author (D. Nicols) about the sack of Constantinople and Hattin and they are both not quite good. Many innacuracies, mediocre descriptions, lack of background, lack of analytical thinking... Perhaps it's just the particular author (even though he's considered one of the top writers that work for Osprey) or the very tight-ass military orientation of those books (when describing an event as Hattin or Constantinople needs a solid background of general knowledge pertaining the particular periods and events).

The plates are top-quality though and fairly accurate. Although in the Ancient Greeks series they have numerous innacuracies (for instance, Osprey's books is one of the most infleuental factors in the spread of the utterly false idea that Spartans fought butt-naked).

The only book I have from Osprey is a large, hardback book about the Boar War, its by Martin Marix Evans. Its alright I suppose, after buying it I found alot of better Boer War books (most of which were Australian in focus, which was more to my interest).

As for the Men at Arms and similar type series, whilst I think the artwork is well done and they look nice, they are a little too pricey for what they are.

My $0.02.

Osprey books vary a lot in quality. The quality ranges from pure crap to great sources of learning. I have some 30 books, a few are really bad, most are mediochre and a handful are great. Of the books I own the worst are written by David Nicolle and Stephen Turnbull. Both authors tend to be careless with facts and in the case of Nicolle - way too fictious and speculative.

The older books tend to be boring (dry texts and unimaginative illustrations) and contain outdated facts, especially if they deal with ancient times. The illustrators and their works also vary in quality - Angus McBride is a great artist but his numerous illustrations tend to be very fictious and historical inaccurate. Graham Turner on the other hand is one of the world's top military illustrators whose illustrations are both beautifuly realistic and historical accurate. Please check out his http://www.studio88.co.uk/ - webpage btw. 

I own around 20 of them, and so long as you don't take them as the definitive work in that field, they are generally quite useful little overviews.  As others have mention, the quality of them varies greatly, and the facts and illustrations can get a bit too fictitious (as a general rule of thumb, if an Osprey illustration or bit of text sounds too wild to be true, it usually is).

As for the authors, I've got to say I'm not fond of Sekunda.  He tends to base excessively wild speculations on too little, or in fact, no evidence and is otherwise inaccurate often.  Though I'll admit some of his theories are at times well supported and intriguing.

The stuff on early imperial China is reasonably good, but the "Medieval" and "Late Imperial" volumes are very inaccurate.

Definitely agree with you on Graham Turner, really great artist IMO.

Very brief and not enough, when the book finish , I feel I red a newspaper, but a good project