ACATS are good but they can have some problems. They take about 2 weeks so there will be no trading during that time. If you are doing investment accounts then it is not a problem. If they are trading accounts then the two weeks may hurt. You must monitor for rejects. IB does not send you an email. The info only appears on the transfer page. If there is a reject you must re-initiate, IB does not.
IB will reject ACATS transfers if:
1) There are any unsettled trades (trades done within three days of the ACATS request). Submit the ACATS request at the close of business on the settlement date, You can try to finesse this by doing it one day earlier and hoping that the other firm does not react instantly to IB's request.
2) You have any penny stocks in your account which IB does not trade in. These are normally "Pink Sheet" stocks. You can check this by going to the various exchanges on IB's web site and seeing if the stocks are listed there.
The transfer time is advertised as not exceeding 10 business days. It will probably be done in 5.
The account is essentially frozen for about 8 - 10 days. I would not have anything in the account that you would be nervous about for 10 trading days.
One other thing - IB has, in the past, not done ACATS stuff over an option expiration weekend as their staff is busy handling assignments and executions. I'd initiate the ACATS request to avoid that weekend. They may have fixed things by now but one never knows.
Jack