If you mean F-S's I thought it was worth owning, one main complain was that there are not line drawnings on different versions and subtypes, but I also own Prien/Rodeike F, G and K book, which is good but a little dated and Valtonen's which both have many good line drawnings, so I didn't need one more book with them. I have also some other books on Bf 109 and on the a/c of the Finnish AF so I can cross-check the info. I'd recomend that you read the links I gave in my earlier message, there is also one more thread, the first one, on the discussions in TOCH on F-S's book on that site. Read those and draw your own conclusion. I used mostly F-S's, P's/R's and Valtonen's books and Raunio's Lentäjän Näkökulma II when I check something on Bf 109Gs.
Juha