I use most of them and all of their platforms. So I'm with FXCM, IB, InterbankFX, and FXDD. But not OANDA.
I would say that IB has good consistent spreads. the most divergence I've seen is about 4-5 during highly volatile periods.
However IB calculates spreads to the 5th decimal point.
i.e. AUD/USD:
bid = 0.84505
ask = 0.84510
So a 1 pip spread for IB is really a half pip spread compared to most bucketshops.
this means that when I say 4 pip spread:
i.e.
bid = 0.84505
ask = 0.84525
it is really only a 2 pip spread due to the fact that they calculate the currency pairs to the 5th decimal place. i.e. FXDD might have a spread of;
bid = 0.8450
ask = 0.8454
where as IB, if you set the quoting out in similar fashion it is;
bid = 0.8450
ask = 0.8452
... notwithstanding that the fact that there is no rounding in my example above.
IB can do this b/c they charge about a $2-2.50 commission per leg (i.e. about $4-$5 per trade overall). Some people might be wary of this but think about it;
if you have 100K and your trades are around $10 for every 1 pip movement and you stand to make 20 pips off a profitable trade then you make $200. $5 is pretty miniscule. Whereas if you trade with a bucketshop and the spreads are 10 pips during highly volatile periods, then you missed out on $100.
For bigger account sizes and large volume trades, $2 commissions are not huge. Your broker has to make a living somehow, and they can either take spreads or commissions, or do a combination of both. I don't mind paying commissions so long as the broker is providing a good service and is committed to improving its infrastructure and technology, and is not in the game solely to trade against you like the bucketshops are.
In terms of execution and data and so forth, I think there are enough reviews for you to figure out where IB stand.
I will say this, whether you choose to use IB or not depends a lot on your trading style and what/how you trade.
If you're just interested in forex solely then I think it is not really necessary to switch to IB, given all the complexities of their system and minimum capital requirements. The learning curve is quite steep with IB compared to say; using MT4 and taking simple trades.
If you're a good trader you will still be able to make money in a bucketshop environment. However if you have a large account size, and trade other things aswell; i.e. global futures, stocks, bonds and commodities ... then IB as a broker becomes a lot more attractive as a value proposition.
Good luck with your trading.