Early in my training, I'd used a trap bar exclusively for dead lifting. When I went to another gym that didn't have a trap bar, I couldn't dead lift nearly as much as I could with the trap bar. It seemed to me that deadlifting with a trap bar transfers poorly to dead lifting with a straight bar, however, the opposite is not true.
For me at least, trying to go from trap bar DLs to straight bar DLs was awkward and yielded poor results, at least for my DL. I don't know if trap bars are a good training tool or not, I've never used them past the point when I moved to a gym without them, even when given the opportunity. I guess they could have been useful because they allowed me to lift a bit more weight than a straight bar, at least before I'd done much heavy lifting.