I have sb'd fx for some time now and I have to say that the sb price tracks the "real fx" prices very well, with little (what some would term as bias) difference other than at extremely volatile times.
I really don't see this as an issue as the nature of the fx markets means that even between "real" fx brokers, their prices can differ by similar amounts anyway. If you look at the sb spreads of say cmc and capital spreads (and probably some others), they are equally as good as "real" brokers. The only way to get lower spreads is via futures whch limits the currency pairs you can trade (eurusd is most liquid future I beleive).
I have said it a number of times on here before but if you want to trade fx (other than futures), the sb companies are as good a bet as the real fx co's.
The only disadvantage with SB imo is that the dealing platforms tend to be less flexible. (My main issue atm is that the sb's do not offer if done orders). However, if daytrading and constantly watching the screen, this also should not be an issue.
I use cmc spreadbet for dealing fx. Yes, their customer service is pretty poor and the dealing platform not the best for fx but they have low spreads (lower than many "real" brokers) and I have never had a problem with fills/ slippage/ bias etc. The only time I have had slippage on an order was last friday when the markets went mental but this was only 4 pips. Not bad compared to some other real brokers (GCI in particular seem to have had real problems judging by another thread on here).
Haven't really traded the dow before but have studied the dow a fair bit and I find fx to be much more tradeable, ie longer trends, plent of volatility etc. There tend to be good moves early morning when european trading kicks in usually 7 - 8 am is when a move can kick off and you can sometimes pick up the start of a move around 1 to 1.30 pm when US trading/ news kicks in.