I used them specifically for pit traded options and they were horrendous. I had to phone in repeatedly to verify the prices I was filled at. Even after those corrections they found at my urging, I'd find adjustments retroactively applied to my account, well after the transactions took place.
Also, the listed option prices are stale, end-of-yesterday at best. I'd have to phone in constantly to know where a position really stood. After a while, their phone reps began to sound irritated. I'd have been happy not to have to call in, but their system generates false margin call warnings based on the stale prices! On straddles, no less, whose margin requirements are fixed!
If you're looking for an electronic markets broker, there are cheaper ones available. XT's only draw for me was their access to pit traded markets, but I found the poor service and executions invalidated any advantage.