I use NYSE Open Book every day and consider it invaluable for my trading style. Everything everyone's said about it is true-- that it lags, freezes, and may not reflect the true book-- but it is very possible to learn how each specialist on a stock likes to use his open book, and thus very useful. Combined with a regular Level II and tape reading skills, I use NYOB to track microterm supports and resistances, to pick entries and exits, to figure out what side a stock's specialist is on... I think that the shorter the terms of your trades, the more useful Open Book will be to you.