I use L1/L2 for every trade I place.
The L2 data for Stocks & Futures comes from a centralized exchange or group of exchanges in the case of NASDAQ. For stocks, there are various L2 services but in effect what you see are all bids & offers for the instrument.
With Forex, there is no centralized exchange, there isn't even a group of exchanges as with NASDAQ. With Forex L2 is not possible for this reason. All a broker like Oanda can show you is some of the picture, which in this case is simply the data related to their clients which is probably .00000000001% of the total market.
As such, it's not really going to do you any good using something like this in my opinion.
Your best bet is to subscribe to a Futures data feed and look at the total bid/offer picture for the corresponding Forex Futures markets. So if you want to trade EURUSD, then you look at L2 on the 6E, at least that way you are seeing the whole market and not a tiny subset.