H1B stamping
I presume you are planning to have your H1B stamped on your passport from a J1 status. I am actually planning to do the same, hence I have been reading a lot about other physicans' experiences (not on this site, though).
As far as I know, there doesn't seem to be much problem at all so long as you have all papers in order and you have not been out of status in the past. I could tell you the problems that I've read: not being able to have an appointment in their home country (for people who originally wanted to go home for the stamping), thus, ending up having to have it stamped in Canada/Mexico, instead; being stuck in another country because for security check purposes, their visa cannot be given right away; or for some reason, the visa got refused.
Do not let these incidences scare you though, because majority of the time, there have not been any problems at all. The consular officers seem to be really great in Canada. Just make sure all your papers are in order ( and hopefully, you have not been out of status before). Even with the unfortunate incidents that I have read, I'm still going to Canada to have my stamping done there as based on how easy it has been for a lot of people, I'm quite assured that mine will not be a problem, either.