IB data isn't so bad, its just that, as mentioned above, it is snapshots, and also the Bid/Ask volume is not correct, so you couldn't use studies that rely on this. In Sierra Charts, there is a setting to download accurate data every 5 seconds. Without this setting, sometimes the bars end up being a few ticks off. But with this setting, the data is very accurate. IB data does also have level 2, and they allow you to have level 2 for 3 different instruments. (I only have experience with futures)
When you upgrade to SC data, its very good. In fact, when the data is reloaded, it matches up 99.99% to the data that comes in live. SC has also now increased the depth of level 2, and you get 30 levels of Bid and 30 levels of Ask on some instruments. Yes, 30 levels, whereas the usual is 10.
IB data has also never once dropped out on me in fast moving markets. You read reports every now and then of people having dropouts, but I can say I never saw it with IB, even in fast moving markets, and even when the market hits the limit down like it has a couple of times over the past year.
The IB data is very cheap, maybe around $10 from what I remember, but you can get this back if you have commissions. The Sierra data is $35 plus whatever markets you need data for, but for the CME package, its $5.25 I think, so works out to about $40. If this extra cost is worth it to you is an individual matter. But the IB data isn't bad at all, you have 10 levels of Bid and Ask for level 2, and is fairly accurate with the proper settings. The SC data is pretty much perfect and fairly cheap as well.